PetMeds® Comfortis Flea Control: Potential Side Effects

Did you know that not all flea control products for dogs are applied to the skin? Many pet owners prefer oral administration of external parasiticides to avoid exposure of themselves to topical pesticides, which is why some dog owners prefer the ease of administering a chewable tablet such as Comfortis. Comfortis can control fleas up to one month on dogs, but it does not kill ticks

Comfortis kills fleas within 30 minutes and lasts for up to one month. In my experience common client concerns include lack of efficacy against other external parasites such as ticks, as well as the need for written prescription from the veterinarian.

One of my concerns with Comfortis is the occasional wide range of side effects including vomiting and/or diarrhea and various neurological reactions ranging from incoordination, trembling, and seizures.  It’s for these reasons that I generally prefer using other flea and tick products in my practice such as Frontline Plus or Advantage II.

However, clients do report mostly high satisfaction with this product, with the exception of these occasionally severe side effects.  It is also important to still treat the environment for fleas, as well as incorporate additional measure to help control other parasites such as ticks if using Comfortis.

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  1. im doing a speech on comfortis and im wondering if its better than a flea coller can you help me

  2. I LOVE DOGGIES HE HE HE AM I CRAZY and i dont want them to suffer of fles my poor darlings so im going to use comfortis is it good or should i use a pet flea collar

  3. Dr. Michael Dym, VMD veterinarianNovember 20, 2011 at 7:25 pm

    Comfortis does work better than flea collars in most cases.

  4. thanks so for whole body flea treatments use comfortis

    but neck treatment you can use flea collars if you want for good results cool any other information for me ?

  5. hi im soooo crazy hey i take drugs do you hey you should see me some time and hang out in an alley together eh whats your phone number and dont worry about your question i love you i know im awsome arnt i

  6. oh well you can get married at the age of ten i love you im asking for your hand in marrige and yes you are gonna be my wife hahahahaha

  7. hehehe yes ill marry you i love you to

  8. this is unacceptable you two i have the right to ban you please be sensable

  9. Dr. Michael Dym, VMD veterinarianNovember 21, 2011 at 8:27 pm

    I am not fan of using flea collars when pets are on other pesticides such as comfortis.

  10. thanks but have you Seen any side effects yourself and has THE CUSTOMERS BEEN UPSET WITH THE PRODUCT

  11. Dr. Michael Dym, VMD veterinarianNovember 22, 2011 at 10:31 pm

    I have not seen many side effects on the few cases I have used it in practice, but have heard and read of side effects.

  12. yes ive heard about seizures and diarrhea but yes thankyou for your information in the future i will use that product

  13. Dr. Michael Dym, VMD veterinarianNovember 25, 2011 at 7:45 pm

    You are very welcome. Spread word to your family and friends a about this service.

  14. Within a few hours of giving dog medicine she became very lethargic. Normally she is very active and plays constantly but not after this. She just laid around for about 24 hours finally started getting some energy back slowly. Wow bad bad smell and horrible breath to go along with this medicine which lasted about 3 days. The worst thing I have ever smelled. Will NOT be using this again.

  15. Dr. Michael Dym, VMD veterinarianMarch 31, 2012 at 3:16 pm

    Occasional side effects with any pesticide may be seen. Thanks for sharing and I agree on not using this again in your animal companion.

  16. Taking my two dogs for flea control. My vet uses confortis. I have heard about bad side affects from this med. What is your opinion?

  17. Dr. Michael Dym, VMD veterinarianJuly 26, 2012 at 1:37 pm

    Oral comfortis is very effective for flea control. Like any pesticide products rare side effects can be seen in some animals.

  18. I gave our German Shepherd Comfortis tonight. Just returned from the Emergency Vet. Within a half an hour after eating the medicine, her back legs went out. She couldn’t walk. I’m sick over this. I’m praying she will be better \in the next couple days\ as the emergency vet said, as the meds get out of her system. They had to sedate her to do X-rays because we thought she’d broken her leg or her hips were in trouble. So, it remains to be seen. No…nothing broken, hips fine. Coincidence? I think not. I’m certain it was the COMFORTIS. Maybe she’s one in a hundred who gets serious side effects, but she’s one in a million to me and I am broken-hearted that she trusts me and I gave her this poison.

  19. Dr. Michael Dym, VMD veterinarianAugust 11, 2012 at 5:00 pm

    Sorry to hear about reaction to the comfortis. Consider alternative topical product and/or natural flea control or

  20. Our dog was only 6 when he did last year from liver failure. We had him on Comfortis for most of his life. Has there been any association between Comfortis and liver problems? Our last dog (different breed) lived to 14 and we were giving him frontline.

    The Comfortis worked great as we live in a hot climate that produces both fleas and ticks. But before getting another dog, I’d like to have some answers to why our dog died at a relatively young age.

  21. Dr. Michael Dym, VMD veterinarianSeptember 13, 2012 at 8:45 pm

    Any topical or oral pesticide has the potential to cause problems long term with organ function but hard to prove or disprove. Also other toxins in environment or food could be involved as well.

  22. No, not this time. We are very careful with our pets. My dog was running for a frisbee before she at this pill…perfect health. Had just had a physical recently. Three years old…in her prime. 15-20 minutes after eating this pill, her legs gave out! ABSOLUTELY NO QUESTION AT ALL. It wore off, thank goodness. Company realized issue. Reports were made to FDA. Company was concerned and helped pay emergency bills, etc. They realize this is a severe possible side effect, but I didn’t. My fault, as the consumer…I didn’t research it enough.

  23. Dr. Michael Dym, VMD veterinarianSeptember 13, 2012 at 9:49 pm

    Thanks for sharing. Sorry for your dog’s experience with comfortis.

  24. I gave my dog comfotis and she immediately started scratching and her skin was very red. i thought she had got into something in the yard so i treated the entire yard with weed killer and pesticdes. She still has the same problem only worse. Took her to the vet today and they say it’s her thyroid. Didnt have any problems until she started taking this. Have you seen any side effects such as thyroid during use of comfortis?

  25. Dr. Michael Dym, VMD veterinarianOctober 12, 2012 at 6:22 pm

    I have not heard of comfortis causing any thyroid problems in dogs.

  26. I am using Comfortis for the 2nd month with no side affects, but limited success. I increased the dosage the 2nd month, because my dog is borderline 40 lbs, still no side affects, but she is still scratching and chewing, however, not as much. I have never had a flea problem until this year and am trying to vacuum and keep the house free of any larvae, eggs, that may be inside. Still
    the scratching and chewing is driving us, both, nuts.
    Any suggestions on anything I can put down in the house and bedding to help kill any fleas that may are hatching after I started treating her? Or should I try a topical flea killer on the dog instead of the Comfortis?

  27. Dr. Michael Dym, VMD veterinarianNovember 11, 2012 at 4:44 pm

    I would try oral program and capstar from 1800petmeds. Also consider flea busters for home which you can find on line.

  28. yes i have fleas and i am sooo itchy

  29. Lol funi just werd

  30. jasmines on more craNovember 11, 2012 at 8:58 pm

    she awsome

  31. derr brain

  32. kayla stop it u r mean
    i hate you 🙁 !!!!!!!!

  33. 🙁 lol ae it was funi

  34. I gave my dogs Comfortis for the first time and one has horrible bloody diarrhea for three days. Vet has prescribed Pana something (liquid) and nothing has changed. Other dog is extremely lethargic and won’t eat. If they do live thru this, I will NEVER use it again and if they don’t make it… I wish I could sue. I am furious and scared.

  35. Dr. Michael Dym, VMD veterinarianNovember 15, 2012 at 12:31 pm

    If symptoms persist over a few days see vet again for more supportive care. Side effects should wear off within a few days.

  36. i gave my dog comfortis and a few hours later she started throwing up and had horrible diarrhea for ages
    comfortis seems to have a lot of problems and i will not be using it again

    you should also tell those silly kids to stop writing those comments on they are taking advantage of your blog !!

  37. Dr. Michael Dym, VMD veterinarianNovember 25, 2012 at 7:54 pm

    Thanks for sharing your wonderful and touching story.

  38. Dr. Michael Dym, VMD veterinarianNovember 25, 2012 at 7:55 pm

    Some pets can react to comfortis. Thanks for sharing your thoughts. Consider natural flea control if chemicals causing problems such as wondercide or cedarcide

  39. i like boys

  40. natasha get off this site im dieing to havee explosive diroheea soon and i need you here to comfort me

  41. I gave my 14 year old AmStaff her first dose of Comfortis almost 24 hours ago and I am happy to report that she did not have any visible side effects and even better than that, she doesn’t have any fleas! When we lived in NY Frontline was all that was needed and it was moreso for ticks. I had never seen a flea before moving to FL. Frontline, baths and herbal remedies were to no avail. Comfortis seems to be working like a charm.

  42. Dr. Michael Dym, VMD veterinarianDecember 10, 2012 at 1:41 pm

    THanks for sharing. Comfortis is usually quite effective orally. Good luck in this tough state of Florida with fleas, etc.

  43. My older dog was given comfortis on Dec 28th after dinner.

    He started throwing up at 2:45am Saturday morning.

    He is so sick right now, and we can’t seem to make him feel any better.

    Have been to the vet 3 times in 3 days…sub q fluids, cerenia
    Homeopathy, etc.
    His liver & kidney #’s are normal waiting on pancreatitis

    Hoping he will turn around…but I am worried sick myself.

    It’s been 48 hours, and its not getting any better.

  44. Dr. Michael Dym, VMD veterinarianDecember 31, 2012 at 11:30 am

    HI Holly: Sorry to hear of your dog’s problems after comfortis. Probably best to consider IV fluids at vet office for a few days until symptoms calm down. What type of homeopathy was given? You may want to consult with a classical homeopath, which prescribes remedies based on constitutional symptoms. To learn more see

  45. I used Comfortis for my 3 German Shepherds. Loved it at first, no fleas whatsoever. After a few months of taking it, all three started having seizures. They all started to refuse to eat it. Took them off the Comfortis, and it took 6-8 months before they quit having the seizures. Now just trying to find something that will keep the fleas away. Going to try the Seresto Collar. Hope it works as well.

  46. Dr. Michael Dym, VMD veterinarianApril 15, 2013 at 10:12 am

    Try topical revolution from 1800petmeds.

  47. I just bought a mini schnauzer (8weeks old) and was told not to use oral flea control as they cause cancer. Is that true? I’ve been giving my Boxer oral flea control for years, she seems fine but this comment scared me.

  48. Dr. Michael Dym, VMD veterinarianApril 18, 2013 at 10:03 pm

    Most oral pesticides such as comfortis are fine to use in most dogs. In sensitive breeds, chronic pesticide use may be linked with chronic diseases, although hard to prove.

  49. the comfortis could be causing the scratching, have you read these other comments? you should.

  50. oh my gosh, I just thought my dog was going to die. she has never had a seizure in her life and I gave her one of these comfortis pills this morning for the first time and she just now had a huge seizure. I really thought she was going to die. and now she can’t hardly get up or stand up. I have been crying my eyes out and I just feel terrible. she also peed all over the floor. it seemed like she was never going to come out of it. I hope she can walk tomorrow. terrible just terrible. I have to take my finals next week and now all I am going to be able to do is think about her.

  51. Dr. Michael Dym, VMD veterinarianApril 28, 2013 at 10:52 pm

    Hi Lorrie. Consider natural flea control such as Also if health issues persist, consider working with homeopathic vet

  52. I have had my Senior Doxie on Comfortis for three years and all year long. i see the relief that my dog gets in a short period of time and she stops scratching and chewing. i have not seen any bad side effects with this oral medication. it is far worse to expose the dog to flea shampoo and then a toxic flea collar and still see fleas. my vet sells me the pills and matches the doseage to her weight. i reccoment this to dog owners.

  53. Dr. Michael Dym, VMD veterinarianMay 5, 2013 at 10:50 pm

    Thanks for sharing.

  54. Waching your dog have a siezure is frightening. They cannot tell us that they feel odd, or their stomach is upset.
    My spelling was terrible from my last posting. Dogs and cats bring fleas into the house and we carry them in on our shoes. The key I think is to stay on top of them from the start. My dog got fleas after I kept a friends dog with me for four days while the owners went to a cutting horse competition. Harley had fleas and I did not think about it. I let him sleep in my bed with me and my dog. After I returned Harley I spotted the fleas on my dog. I did the flea shampoo thing and did not know Comfortis existed. My neighbor came over and we sat on the floor picking fleas off of my dog and dropped them in a cup of water. Fleas cannot swim so they sank to the bottom of the water cup. I am looking for a natural flea spray that I can use on my dog and on my bed. Any ideas?

  55. I have heaps of fleas please help me i like little children

  56. Dr. Michael Dym, VMD veterinarianMay 6, 2013 at 10:51 pm

    Try flea free from 1800petmeds Another great natural product I have found for fleas is

  57. I just purchased Trifexis, it’s a monthly tablet dogs/cats take to kill fleas, protect the heart and kills any intestine worms. Gave both my dogs this last night, checked them this morning and the fleas are DEAD!!! Amazing product. You have to be careful with the others because they kill the flea larva but not the flea…the flea will continue laying eggs and you will never get rid of them. Trifexis is expensive but well worth it.
    This is where I bought mine from

  58. Dr. Michael Dym, VMD veterinarianMay 8, 2013 at 11:24 pm

    Trifexis is excellent product but do need written script for it from vet. Available economically at 1800petmeds.

  59. The first few times that I pilled my Doxie Gretchen with Comfortis she gobbled the tablet down like a starved animal. I had to start asking her to do something for her “treat” and that lasted..a while. I progressed to wrapping the pill in cheese. as she watched. That worked once. I bought liver sausage and stuck her pill in that for a few months. It has gotten to the point that her and I struggle to get the pill in her mouth and I hope she swallows it. She gets a real treat if I do not see her hack the pill up onto the floor! I have wormed a horse with less trouble. I end up all sweaty and my hair sticking up. She is 15 years old and thinks she is Alpha to the planet. What gives?

  60. Dr. Michael Dym, VMD veterinarianMay 9, 2013 at 12:27 am

    I am concerned that her body may be reacting to the comfortis pill when I see a pattern like this, especially in a breed like a doxie which is very sensitive to drugs, chemicals, vaccines, etc. She is probably learning through repetitive doses that she does not feel so well after taking it, which can be seen with any pesticide in elder animals. Maybe consider an alternative flea control product in pet this age, even a natural option like flea free from 1800petmeds.

  61. I gave my 2 dogs and 2 cats comfortis 2 weeks ago. One cat threw up, the fleas seem to be gone but today one dog started having seizures ..4 so far!! I’m just sick about it !! This was a totally healthy dog!!! Does she have a permanent problem or will it go away ??? What can I do for her ??

  62. Dr. Michael Dym, VMD veterinarianMay 23, 2013 at 11:03 pm

    I have seen rare neurological reactions to any of the oral or topical pesticides. hard to say if seizure disorder was caused by product or just a pre existing tendency was already there. Best to see vet for exam and evaluation to see if anticonvulsant meds may help. Also could consult with a homeopathic vet to try and raise level of health over time. To learn more about homeopathy see as well as my website

  63. I just gave my dogs Comfortis today and within an hour 2 of them started throwing up, not just once, but 5 or 6 times (each). I wish I had never heard of this medication. I have also read reviews and experiences where many dogs even died from Comfortis. My recommendation: Do NOT give this to your dogs.

  64. Dr. Michael Dym, VMD veterinarianJune 7, 2013 at 5:43 pm

    HI Sue. Sorry foryour experience with comfortis. I certainly would not use it again in your case.

  65. My senoor Doxie does well on Comfortis however I am wondering if she is building up a tolerance to the drug. I find a flea or two on her. i purchased flea busters and should get it in a few days. I also purchased an organic flea powder that can be used on my dog and her bedding. She starts her day with a benedryl type pill that gives her relief from the itching. My Vet says that she is allergic to the flea bites. I have a topical spray that I use to stop her itching in one area. I am interested in massage therapy for my senior doxie….just because I love her. BTW flea collars are highly toxic!

  66. Dr. Michael Dym, VMD veterinarianJune 8, 2013 at 1:10 pm

    Thanks for sharing. Also consider natural flea product like

  67. I gave both my dogs comfortis but for some reason only lasted about three weeks then I seen the dogs scratching and spotted fleas on them. Did I maybe not give right dosage I have a bichon fries and an English springer spaniel

  68. Dr. Michael Dym, VMD veterinarianSeptember 9, 2013 at 11:29 pm

    in some cases of heavy infestation, pets need to be dosed sooner than one month. Also remember to treat the home for fleas as well.

  69. We have found just the opposite using Frontline or Advantage, both cause seizures for our Aussie Shepard so we are trying Comfortis for the first time, we’ll see how this plays out

  70. Dr. Michael Dym, VMD veterinarianSeptember 24, 2013 at 6:52 am

    Every pet is an individual with unique sensitivities.

  71. I gave my 4yr old Chihuahua her first dose of Comfortis @ 4 pm yesterday. She got rather hyper and paranoid … She followed me everywhere. She also ate a lot. Instructions said give with a full meal but she kept eating afterward. Today she had a small normal consistency poop, and this evening some diarrhea. Otherwise she is active and happy. I hope it works since she gets FAD really bad every year when it gets warm and we have to go to the vet for Simplicef and Temeril-P.

  72. Dr. Michael Dym, VMD veterinarianOctober 3, 2013 at 11:45 pm

    Every pet different with oral or topical pesticides. If side effects occur again next month, then best to change to different preventative such as program or topical revolution

  73. Can you give your animal a flea bath on top of comfortis. ?it has been about 45min since I gave it and the poor thing has tons of fleas.

  74. Dr. Michael Dym, VMD veterinarianNovember 19, 2013 at 6:32 pm

    You can give flea bath at same time as comfortis.

  75. when in southern california, vet prescribed Comfortis for 10 lb and 15 lb chihuahua mixes. i was amazed how much they scratched soon after application, and was told it was the fleas coming out of their skin… already dead… and that this would kill adults and larvae and eggs alike. i put the pill at the back of their tongues and held their muzzles shut until i knew they’d swallow it… pretty easy with the learned technique, and very clean.
    … since then have moved to the mountains of arizona, where vet prescribes topical Frontline. i’m not sold on this product. just returned from visit to the beach in so cal, and came home with poor itchy pup. an oatmeal bath has not eased his discomfort.
    … it’s been only 2 or 3 weeks since last application of Frontline. i’m here to order Comfortis once again. i’ve never experienced side effects mentioned by others (my sincere sympathy), and have visible results within hours of dose… even though it’s a bit disconcerting to see them scratch like crazy for a while.

  76. Dr. Michael Dym, VMD veterinarianMarch 17, 2014 at 6:05 pm

    I dont have problem with comfortis in most cases. On occasion side effects can be seen with any pesticide. You will need written script from your vet to order the comforts

  77. I have an Epileptic 3 year old Aussie whom I can not give him any flea meds that contain Comfortis. He has never had a Flea before but lately I have noticed a couple and although I am quick to catch them, I know the risks & dangers of having even 1 tiny flea. I have been looking everywhere online, in pet stores as well as asking my VET. and I can’t seem to find a resolution. I need help finding a natural flea product where it won’t cause my baby to go into seizures every month. Can anyone Help please? Thanks

  78. Dr. Michael Dym, VMD veterinarianApril 26, 2014 at 3:58 pm

    Consider working with a holistic veterinarian on improving your dog’s overall health so not so attractive to fleas. See my website for more information. Many holistic vets do offer phone consultations. An excellent natural flea product is

  79. i had no adverse reactions and much less scratching after this mid-april dose of comfortis. i still prefer it to frontline. will report back if any bad experiences… otherwise, i’m sticking with comfortis because it seems to work great for my pups.

  80. Dr. Michael Dym, VMD veterinarianApril 28, 2014 at 2:47 pm

    Thanks for sharing

  81. Add me to the list of having a dog who quite likely had an adverse reaction to Comfortis. My 13 year old Border Collie had severe vomiting and tremors approximately 5 days after receiving Comfortis. The tremors became uncontrollable and progressed to the point where she could not walk. We really thought we were going to lose her. After rushing her to the ER to be stabilized and monitored, along with a follow-up consult with a neurologist, we were unable to make a positive dx. The one new factor in her life was Comfortis. After taking her off Comfortis, she is now completely normal 3 months post the tremors/neurologic episode. Was I informed by the neurologist of a possible connection to Comfortis? Nope, he only wanted to do a spinal tap and an MIR….Positive conclusion? Nope. Will I ever give her Comfortis again. H NO!!!

  82. Pingback:

  83. We gave our lil 20 lb dog comfortis two days ago shes normally very active shes only 4 yrs old. She slept and layed around for two days!!!! She had a bad odor coming from her skin we are NEVER giving her this again!!!!!

  84. She was also trembling.

  85. Dr. Michael Dym, VMD veterinarianJuly 4, 2014 at 6:31 pm

    Sorry to hear about this Mandy. Hope she feels better soon.

  86. I gave both my dogs a cap star….then bathed them a few hours later, then before bed I gave them both a comfortis. 6 hours later the little one is fine! The larger female has been out twice and acting like she can not get comfortable. Has this can’t get comfortable feeling for over one hour. Plus during her time of going outside she has had 3 bowel movements. She is just not acting right.

  87. Dr. Michael Dym, VMD veterinarianSeptember 24, 2014 at 10:34 am

    Could be reaction to too many meds at once. She should be fine in a few hours. If symptoms persist or worsen, see vet for evaluation

  88. I gave my 2 yo dog comfortis 2 weeks ago. After giving it to her I noticed she was having a difficult time walking, even became paralyzed for a couple hrs. She normally is very active and intergetic. It’s been 2 weeks and she’s still having difficulty getting around. I have taken her to the vet but they brush it off as being a spinal issue and tell me comfortis won’t cause these complaints. know this is a side effect, but was wondering if she will always have trouble now, or if maybe this will wear off with time.

  89. Dr. Michael Dym, VMD veterinarianOctober 14, 2014 at 3:59 pm

    Hi Tia. Hopefully it will wear off with time. I do agree with you, any topical or oral flea/tick pesticide may cause neurological issues in some sensitive patients. I would consider consulting with a holistic vet. to learn more about homeopathy see the booklet on the website and my website . Many homeopathic vets do offer phone consultations.

  90. I used comfortis on my golden puppy 4 times starting from 16 weeks old. The first 3 times he was absolutely fine. Last time he got the pill was 5 days ago. The next morning he started vomiting with yellow liquid, he stopped moving, eat and drink, we took him to the vet the next day, they made fluids, ton of injections and gave medication for using at home. He felt better right after procedure but he still didn’t drink or eat and started throw up in the evening. We took him to the vet again, they did the same and the result was the same. He’s continuing vomiting and refusing eat and drink. I don’t know what to do, it’s 4th day without food, so I literally push can food in his throat, and in couple ours he throws up this food and medication. Veterinarian obviously can’t help, I know for sure that it’s happening because of comfortis cause that was the only difference in his routin. Please help me somebody who had this problem! What else can I do to save my puppy? He’s dying in from me

  91. Dr. Michael Dym, VMD veterinarianOctober 19, 2014 at 3:16 pm

    There may be another condition in a young puppy like this such as an undiagnosed foreign body. I would ask your vet to send the x rays out for a board certified radiologist to read. Barium swallow x ray studies may also be helpful here. If not already done, I would recommend a FULL CBC/chemistry blood profile to make sure nothing else going on. If your vet stuck, then referral to vet internist in your area or specialist may be worthwhile.

  92. DO NOT GIVE your animals Comfortis or Trifexis (which has Comfortis). My dog ended up in the ER and had to be put on oxygen. If i had not taken him, he would have DIED over this medication! He could not breathe and was trembling. We took him to the ER and he passed out on the table and had to be rushed to the back for oxygen and add’l meds to save him. Wish I had read reviews before giving it to my dog! Look at all sites for all bad reviews on this medication and choose something else! Your dog or cat will love you for it!!

  93. If cat only threw up over an hr after ingestion will it still work? I hope some got into his blood. He itched like crazy for an hr or two after ingestion and maybe longer. Then some time during our sleep he threw up lots of kibble and fur. This morning he itched and shook once but seems settled. I am really hoping the vomit was from the sensation he felt after it was in his blood and NOT that the pricey pill was lost in the vomit on the floor! Expensive flea collar, Revolution, nor FrontLine worked.

  94. Our Shih-tzu had a reaction to topical flea meds, so vet prescribed a prescription flea spray that we used for 5 years. Then we moved to a hot climate and she got hook worms, so the vet put her on Comfortis. 6 months later at her regular 6 month check up, she now has liver disease! I dont think this is a coincidence. Nothing else has changed… we will be going back to the spray and now have to feed her special food and vitamins. 🙁

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