Do heartworm & flea meds interact with dog's potassium bromide?

Great question! As far as actual drug interactions, there is no concern with administering potassium bromide with any oral or topical heartworm or flea/tick preventatives. Some oral flea and tick preventatives (such as Nexgard and Comfortis/Trifexis) state to administer with caution in dogs with a history of seizures. That said, these results come from a controlled studies where several of the dogs had a history of seizures prior to the administration of these medications.

On a personal side note, I give my own 10 year old dog Nexgard and Heartgard as well as potassium bromide for seizure control without hesitation. I do not feel that the studies are controlled well enough to indicate a causal relationship between Nexgard and seizure activity. That said, if you have major concerns, you can always use a topical flea/tick preventative, such as Advantix.