Caffeine is the most widely consumed psychoactive substance in the world. It is highly addictive and causes unpleasant withdrawal symptoms (headaches being the most common) if you stop drinking it suddenly. Most people get their caffeine fixes from coffee and tea, but it is also present in cola drinks, energy drinks and chocolate (the darker the chocolate, the more caffeine it contains).

Caffeine is a strong diuretic and can cause insomnia, tremors and reduced fertility in men. It's bad news for pregnant women too.