Can I Give Benadryl To A Dog Taking Heartworm Medicine?

You have not mentioned in your question which heartworm medicine is being used in your dog? Heartworm medicines can be for curative use and preventive use. Anyhow, Effects of Benadryl can be affected by following drugs.

1. Monoamine oxidase inhibitors
2. Sleeping pills and anxiety medicines
3. Medicines to treat depression like SRI, SSRI, tricyclic compounds
4. Sedative medicines

You can also discus this with your local vet.