My 2 year old boxer who has had severe allergic reactions to vaccinations had a similar reaction to Frontline last week so I just wanted to post this in case it might help someone else. Within a couple of hours of application she began to cough, choke and hives sprung up. I washed off the frontline, gave her benadryl and within a couple of hours she was much improved. 12 hours later she was fine without any follow up benadryl.