Alcohol
Accepted as a drug and still in very common use. It's usually taken for a stimulant (something that revives you, give you more energy etc.), but it actually a depressant. Some types (such as red wine, which helps stop heart disease), can be helpful in small doses, but large amounts can lead to liver problems, and drink shouldn't be something you take regularly in large amounts. Addictions can lead to problems not only with you, but also the people around you who love and care for you.
Smoking
Another common one. Nearly everybody knows this is bad for you, but most people can't give up. In school I vividly remember the teacher writing up a load of chemicals on the board, and I figured that they were uncommon, nobody knows what they are etc., but when he told us what they were, I don't think I was the only one shocked. Cigarettes contain nail varnish remover, light fluid, paint thinner, things like that as well as nicotine etc.! As with alcohol, smoking is only psychologically addictive - your body doesn't actually rquire the nicotine, it's your brain telling you you need it. Which you don't. But ask any cigarette smoker who isn't a science teacher and they'll tell you they are physically addcited to cigarettes, which of course they aren't. Smoking also leads to many different types of cancers and diseases.
Cannabis (weed, grass, etc.)
Another psychologically addictive drug. Although it can help cure other diseases, it messes with you mentally and you end up losing your grip on reality, pretty much. Sure, it makes you feel good, whatever but is it really worth that? As with other drugs, it costs, it's illegal, and it's stupid!
Tea/Coffee
Slightly surprising eh? Some of the milder effects are what people have mentioned on the tagboards in 'Your opinion' - stains your teeth, gives you withdrawal symptoms and other things like that. The worst effect though is probably what happens to the inside of your body. If you're old enough, you've probably learnt about veins, arteries and capillaries in science lessons. Now capillaries are only one cell thick. You need a powerful microscope to even see one cell, so that's how small it is! Coffee and tea gradually reduce the size of your capillaries, so before you make a cup think again! This is also the same for any drinks with caffeine in them, so be careful :D