A drug is any substance (with the exception of food and water) which, when taken into the body, alters the bodys function either physically and/or psychologically. Drugs may be legal (e.g. alcohol, caffeine and tobacco) or illegal (e.g. cannabis, ecstasy, cocaine and heroin).
You may call them simply doctors. But most doctors have extra expertise in one type of medicine or another. In fact, there are several hundred medical specialties and subspecialties. Here are the most common types of doctors you'll likely see.