It frustrates me that this belief is still out there. ALL heartworm preventatives are on the MDR1 problem drug list, but at doses used for heartworm preventative ivermectin is perfectly safe for MDR1 mutants. It's at higher doses used for other problems such as demodex that it is a concern.
From the
WSU website:
"Ivermectin (antiparasitic agent). While the dose of ivermectin used to prevent heartworm infection is SAFE in dogs with the mutation (6 micrograms per kilogram), higher doses, such as those used for treating mange (300-600 micrograms per kilogram) will cause neurological toxicity in dogs that are homozygous for the MDR1 mutation (mutant/mutant) and can cause toxicity in dogs that are heterozygous for the mutation (mutant/normal)."
"Selamectin, milbemycin, and moxidectin (antaparasitic agents). Similar to ivermectin, these drugs are safe in dogs with the mutation if used for heartworm prevention at the manufacturer's recommended dose. Higher doses (generally 10-20 times higher than the heartworm prevention dose) have been documented to cause neurological toxicity in dogs with the MDR1 mutation."