MDR1 – A Generally Safe Drug Could Turn Deadly


Last weekend I attended my cousin’s outdoor wedding in the horse capital of the world, Ocala Florida, and was thrilled to see that they included their Collie in the wedding party. “Barkie” is certainly part of the family, so why not include him in the festivities? As I sat there enjoying the beautiful ceremony to one side and the setting sun to the other, a thought suddenly entered my mind… I wonder if Mark knows about MDR1. My cousin Mark is also a pharmacist and he works in a chain retail outlet dispensing medication for humans.

The following morning I was invited to a Sunday Brunch with the wedding party and during my congratulatory discussions I managed to ask my cousin about “Barkie” and the current heartworm preventatives he is on. I was aware that Mark does not deal enough in veterinary pharmacy and may not have been familiar with this potentially impaired gene that affects many dogs in the herding breeds including collies like “Barkie,” sheepdogs, shepherds and mix breeds containing a herding group. Thankfully, “Barkie’s” veterinarian was familiar with MDR1, had discussed this with Mark’s girlfriend at the time (now his wife) and knew to prescribe a heartworm prevention that is safe for him.

Many dogs mainly in the herding group have an abnormal MDR1 (Multi drug resistant 1) gene. Having this defective gene prevents drugs from being moved out from the cell resulting in high levels of the drug due to accumulation. Toxic levels, sometimes life threatening, may result when certain drugs accumulate in the body. Although this defective gene is present mainly in dogs within the herding group, a test can be performed to determine whether your dog has this defective gene and also differentiate whether there is one defective gene or two defective genes. Dogs with two defective genes are even more susceptible to drug toxicity therefore even more extreme caution should be taken.

Your PetMeds pharmacist can help with questions you may have about your pet's medications

The most common drug that is at risk of accumulation in dogs with MDR1 is ivermectin. Ivermectin is in products such as Heartgard and Iverhart and is used for the prevention of heartworm disease. Accumulation of ivermectin may cause life threatening central nervous system effects. Certain cancer drugs may also accumulate, causing very bad adverse reactions. Digoxin is a cardiac drug that may cause very dangerous toxicity to the heart if it accumulates because of the MDR1 gene mutation. Other drugs such as morphine, codeine, and acepromazine may accumulate causing loss of consciousness, central nervous system depression, and respiratory depression possibly leading to coma or death.There are many other drugs that may be dangerous in pets who suffer from this gene mutation. Your veterinarian is the best individual to decide whether your pet is at risk, order the proper laboratory test, and let you know which pet medications need to be avoided or have their dose reduced.

There are many other reasons for a particular dog to be susceptible to drug accumulation besides having an abnormal MDR1 gene. Cell enzymes, disease states, other medication taken are among the many reasons that may also impair excretion. In order to avoid potentially toxic accumulation of a particular drug, it is extremely important to have a good relationship and discuss every concern that you have with your pet’s veterinarian and your pet’s pharmacist. If you have any questions about any medication your pet is taking, the pharmacists at 1800-Pet-Meds are always more than happy to help answer them for you.

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  2. robert cavan dvmMay 9, 2013 at 9:15 pm

    many years ago, When working with horses in Colorado I had to treat a few that were injured in a truck accident before destroying one two were in svere pain and I had to use 30mg dilaudid injectable tablets that came in a sealed bottle of 8 tablets today 30 ysafter retirering my granson who with his wife are in the same feild which had given so much pleasure.They feel bad that they dont or can”t access such medication .I do not like the computers that they are forced to use however, progress I wonder. Thanks

  3. what is a safe heartworm medication for dogs with the mdr1 gene? i have ab aussie, he’s taking iverhart which is obviously ivermectin. hes never had an issue that I’m aware of. he does have bubblegum seizures occasionally ever since running into a palm tree as a puppy. hes 12 years old now. Should i chanfe his heartworm meds? and if i do, to what? since he hasn’t had any reaction I’m assuming hedoesn’the have the gene but I’m terrified to give it to him now. esp since he is old. any suggestions as to other heartworm meds without ivermectin?

  4. Dr. Michael Dym, VMD veterinarianJanuary 9, 2014 at 1:37 am

    I would be careful with any heartworm meds in this breed. Topical revolution(selamectin) may be worth looking at.

  5. We have over 50 dogs at a time in rescue and sarcoptic mange is a big problem. We cannot afford expensive treatments. We do treat some of the dogs with Ivomec but it cannot be used on herdng breeds. Do you have a suggestion what can be substituted for Ivomec.

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