A drug is any chemical you take that affects the way your body works. Alcohol, caffeine, aspirin and nicotine are all drugs. A drug must be able to pass from your body into your brain. Once inside your brain, drugs can change the messages your brain cells are sending to each other, and to the rest of your body. They do this by interfering with your brain's own chemical signals: neurotransmitters that transfer signals across synapses.