According to the WHO, a drug is a substance that can change how a living organism works. Food is usually not seen as a drug, even though some foods may have such properties. Most of the time drugs are taken to treat a disease, or other medical condition. An example for such drugs may be Aspirin or Paracetamol. These are usually given to treat fever, as well as certain infections. If such drugs are taken over a longer time, they are usually prescribed by a doctor. Other drugs are taken for fun, because of the effect they have. Some of these drugs taken for fun are better accepted by society than others. Having or taking certain drugs may be illegal, in certain countries.

Drugs that are taken to treat a disease or condition are usually called "therapeutic", drugs that are taken for fun are called "recreational" drugs.

Drugs may have other effects than those wanted. Such effects are generally known as side-effects.

Drugs act differently in different amounts. It is therefore important to take the right amount. The amount of the drug taken is called a dose. Aspirin is often prescribed against fever, or as an analgesic. One of the side-effects of Aspirin is that it makes the blood thinner. For this reason, it can also be used to prevent strokes, or heart attacks - in a much lower dose than the one used to treat fever, though.

Taking too much of a drug ("too high a dose") can cause sickness or even death.  When a doctor says how much of a drug (the right dose) to take, only that amount should be taken.  It is very important to keep taking the drug until the doctor says to stop because someone can feel better but still be sick.  When a doctor says what drug to take, how much of it, and for how long, it is called a prescription. Drugs may cause addiction. Some drugs are illegal because they are very addictive. These drugs are really expensive, too. It can also be sold illegally from drug sellers which may cause serious results

Recreational drugs
Recreational drugs can sometimes be bad for someone's health even if they make them feel good.  Alcohol can also be dangerous if the person drinking it drives a vehicle after drinking. This is called drunk driving.  Many countries make this against the law.  Common recreational drugs include:

Alcohol - In beer, wine and hard liquor
Nicotine - In cigarettes
Caffeine - In Coffee and cacao (See figure)
Cannabis - Smoked like cigarettes, or baked in something to eat, like a cake or brownie
Methamphetamine (also known as Crystal Meth) - Can be smoked, eaten, injected, or inhaled
Cocaine - can be snorted, injected or inhaled (freebasing)
Heroin - can be snorted, injected or smoked in its freebase form.

Certain recreational drugs also have uses as a therapeutic drug. Heroin can be used as an analgesic (a painkiller) and methamphetamine is used to treat narcolepsy or ADHD. Recreational drugs can be dangerous, certain countries have made it illegal to have or take them outside a medical context. Most of the time, this is because they can be addictive or that they are very dangerous when combined with other drugs. That way, Cannabis-based products are legal in the Netherlands, but illegal in many other countries.

Alcohol and Nicotine are legal almost everywhere, but there are a lot of taxes on the sale of these. In other words, they are expensive to buy. Being expensive to buy because of taxes has two benefits; one is that the taxes collected from the sale of the drugs goes into helping to pay for assistance the drug user may experience later. A smoker, for example, may develop cancer later in life and need treatment at a place that is granted a certain amount of tax money from the government to keep it operating. Another benefit to the taxing of "legal" drugs, is that the high price may stop people from beginning the habit in the first place, which will help them avoid the negative medical or addictive side effects of the drug. In some cases, not everyone is allowed to buy these drugs unless they are a minimal age to do so. The age at which someone is allowed to buy a legal drug is most often decided by the government of a country, state or province, and is usually the age where a person is considered by the law to be an "adult". Some of the most common ages this happens is at 18, 19 or 21 years of age.

Therapeutic drugs
Therapeutic drugs are also called medicinal drugs, medicine, or medication. Doctors will give therapeutic drugs to someone who is ill to make you better. You can buy these drugs at a pharmacy.  Some drugs need a prescription to be bought, other drugs do not. Drugs that do not require a prescription are called "over the counter" medications. There are lots of these drugs for different illnesses.  These include:

Aspirin - For pain
Paracetamol - For pain
Penicillin - For infections
Prozac - For depression
Diazepam - For anxiety
Antibiotics - For killing bacteria (germs)

References