Have Cat Questions? Ask a Cat Vet Online.
I just came online and saw your question. My name is Rebecca; I am a veterinarian. I am sorry you did not get help from an expert until now.
I am not able to call you, only answer questions here. I did research on the two ingredients in Comb Guard, and your cat got about twice the recommended dose of both. The spinosad might make him vomit at this dose, the milbemycin should not affect him. Cats given 10X the recommended dose of milbemycin usually are not affected. So both of these drugs are pretty safe with a one time overdose.
How is he doing?
How is your cat doing?