Dr Jules Eden, dive medicine specialist and founder of e-med, answers divers' questions - as published in Sport Diver magazine:
Q - I am going to Grenada to do my Open Water referral dives -when I go on holiday I take Zirtek antihistamine to prevent a heat rash - am I ok to dive and take them?
A - Some antihistamines sedate. Others don't according to the packet. But in my experience it's a personal thing. If you find zirtek makes you tired, then do not use it when you dive. If you are OK, fine and alert enough to respond to emergencies on it then great, you're in the blue.
Alternatives are heat rash powders or just wearing loose cotton clothing.
Tip for Grenada. Forget the whale watching. A more claustrophobic, vomit inducing, staff not bothered, back too late for dive times, no whales at all experience, I have yet to find. Mind you I've never been on one of those "fun" Sharm Bus Stop dico-cruises yet!