People complain of symptoms, and think they have some terrible disease. In reality, it's a hangover. The most common trap is if you are staying somewhere on an all-inclusive deal, so you make a pig of yourself at the buffet and drink far more than you are used to, like Monty Python's fat man in The Meaning of Life (pictured below).
Drinking doesn't just give you a hangover, it makes you do things that you wouldn't otherwise, which causes all sorts of accidents. Furniture on cruise ships is often very heavy or bolted down. People don't expect that. If you collide with a bar stool, you might end up with a broken leg.
Drugs are always dangerous, but on holiday it's even worse. For one thing, you might not have access to the correct facilities to deal with an emergency. Also, it can be harder to know what they have been cut with, or if you've been given dummies. The same is true of prescription drugs. People think it is a good idea to stock up on cheap generic drugs in foreign countries such as Malaysia. But the standards might be different, so steer clear.