Certain drugs can cause adverse reactions in certain dogs. Use caution and consult a vet when drugs like heartworm medications are involved. If you are buying medications such as flea medications on your own, make sure to read the label to see what the drug's active ingredient is.

The MDR1 Mutation

While not a blanket statement for every single dog, collies and other herding breeds seem to have a higher rate of issue with the drug Ivermectin. This is due to MDR1, a genetic mutation that is common in those groups of dogs. MDR1 stands for multi drug resistance 1. In layman's terms, this means that the dog's body does not properly utilize the drug. However, just because a drug is not effective, does not mean that the drug is not there. This is where the problem lies. See, Ivermectin is a poison, it is used to kill bugs and parasites, so what happens when you have poison in your body that for whatever reason your body refuses to make use of? Well, you end up poisoning yourself.
If you're curious as to why that is, in more complex terms, the MDR1 mutation mutates a protein. The protein, known as P-glycoprotein, appears in either lesser numbers or is fully absent in dogs that possess the MDR1 mutation. This same protein is in charge of taking Ivermectin or similar drugs and delivering them throughout the body. Without it, the Ivermectin is just waiting at the train station until it overstays its welcome.
Toxicity symptoms set in soon after, which consist of neurological signs given that the Ivermectin train station is located in the brain. Signs include tremors, loss of coordination, seizures, coma, and death.
It's better to be safe than sorry, make sure to express your drug concerns to your vet so that they can pick alternative medications or adjust your dog's dosage accordingly.

Is Ivermectin the only drug I need to watch out for?

If your dog has the MDR1 mutation, or they belong to a breed that the mutations is common in, there are a few other 'class A' drugs that should not be administered.
These drugs include, but are not limited to,
and their branded counterparts.

Shouldn't my vet know this?

Your vet may already be familiar with this concept, but at the end of the day you are the best advocate for your pet. When I came in for my first vet visit the vet tech already knew about ivermectin, however she was unaware of all the intricacies to the point that she and the vet asked me what medications I wanted to give my dog.

What medications did you suggest?

So, none of this is sponsored in any way, just what I extrapolated from the information that Kazoo's breeder gave me.
For heartworm Kazoo started with Interceptor, I wanted to do Bravecto, however the way the dosages work meant that she would outgrow the weight class, so my vet substituted Interceptor. For flea and tick my vet prescribed Nexgaurd and there were no complications with either.
Drugs that are safest for collies and other dogs with the MDR1 mutation are,