What are anabolic steroids?

In one line

Anabolic steroids are man-made drugs modeled on the male hormone testosterone that build muscle and produce male body traits.

What are anabolic steroids?
The first name that comes up in gym talk about 'drugs'
When people at the gym talk about 'drugs' or 'steroids', they usually mean these. Among the drugs people use to build muscle, they are the most widely used and the most studied. The name is familiar, but what exactly are they, and what do they do in the body?
In short
Man-made drugs modeled on testosterone
Anabolic steroids are man-made drugs modeled on a substance called testosterone. Testosterone is made in large amounts in the male body; it builds muscle and creates male traits such as a beard and a deep voice. In Korean they are also called 'protein-building steroids'.
What is a hormone?
A substance that carries signals through the blood
A hormone is a substance made in one part of the body that travels through the blood and sends a signal to cells somewhere else. Testosterone is the main male hormone, and in males it is made mostly in the testicles. Female bodies have it too, but in much smaller amounts.
What the name means
'Anabolic' means building, 'steroid' is a shape
'Anabolic' refers to processes that build large body substances from small building blocks. 'Steroid' is a family of substances that share a shape made of 4 rings joined together. Testosterone has exactly this shape, and these drugs copy it, which is where the name comes from.
What they do in the body
They fit a cell's 'lock' and switch on a signal
Cells have spots where only a certain hormone fits; these are called 'receptors'. Like a key in a lock, male hormones fit the 'androgen receptor' and switch on a signal. Anabolic steroids fit this receptor too, switching on both the muscle-building signal and the signal that creates male traits.
Medical use
Originally, medicines that treat illness
Doctors prescribe anabolic steroids for conditions where the body lacks male hormone, for puberty that comes late, and for muscle loss caused by disease. Using them without a prescription to build muscle is 'misuse'. Misuse means using a drug the wrong way or using too much of it.
The body stops making its own
When it comes from outside, the body makes less
When there is plenty of male hormone in the blood, the brain turns down the 'make more' signal it sends to the testicles. So when a drug brings in a lot of male hormone, the testicles make less on their own, they shrink, and sperm count drops. Even after stopping the drug, the body is often short of male hormone for weeks to months.
Heart and blood vessels
Users caught in testing: about 3 times more heart attacks
Anabolic steroids lower 'good cholesterol' and raise 'bad cholesterol' in the blood, so fatty deposits build up in vessel walls. People caught by gym drug testing had about 3 times as many heart attacks as people of the same age and sex. A heart attack happens when a vessel that feeds the heart muscle gets blocked.
Other body changes
From liver and skin to body shape
Cases of liver tumors and blood-filled pockets in the liver have been reported, and acne is common. Males can grow breasts or lose hair; females get a deeper voice and more body hair. In males, hair loss does not go away even after stopping the drug. Breast growth was back to the starting level one year after starting in one study, and if it lasts, it is removed by surgery.
Hard to quit
About one in three develop 'dependence'
Anabolic steroids do not make people drunk or high, but they cause 'dependence', where a person cannot stop even while knowing the harm. About one in three users is estimated to develop dependence. Stopping often brings depression, which often leads people to start again.
Common misconception
Are they the same as steroids in skin creams?
No. The steroids in skin creams and asthma medicines are 'corticosteroids', modeled on hormones that calm inflammation, the swelling and redness of the body. Anabolic steroids are modeled on male hormone and build muscle. They share a name, but they do different jobs.
Summary
Drugs modeled on male hormone that build muscle
Anabolic steroids are drugs modeled on the male hormone testosterone; they build muscle and create male traits. They are different from the corticosteroids used in skin creams. Related terms: testosterone, androgen receptor, SARMs.