What are aromatase inhibitors?

In one line

Aromatase inhibitors are drugs that block the enzyme that turns male hormones into estrogen, lowering the amount of estrogen in the body.

What are aromatase inhibitors?
A breast cancer drug that comes up in steroid talk
Among people who use steroids, the drug name 'aromatase inhibitor' often comes up. Yet this drug is actually a breast cancer medicine. What exactly is it, and why does it come up at the gym?
In short
A drug that blocks estrogen production
An aromatase inhibitor is a drug that blocks 'aromatase', which the body uses to make estrogen, and so lowers the amount of estrogen. It was made to treat breast cancer in women past menopause.
What is estrogen?
The main female hormone, also found in men
A hormone is a substance made in one part of the body that travels in the blood and carries a signal to another part. Estrogen is the main female hormone, but men have it too. In men, when the balance between male hormones and estrogen is upset, changes such as breast growth can happen.
Enzymes and aromatase
The worker that turns male hormones into estrogen
An enzyme is a protein that helps turn one substance into another inside the body. Aromatase is one such enzyme: it turns male hormones into estrogen. It is found not only in the ovaries but also in the testicles, fat and brain, so a man's estrogen is also made through this enzyme.
Use as a medicine
Treating breast cancer that needs estrogen to grow
Some breast cancer cells need estrogen to grow. By lowering estrogen, aromatase inhibitors slow or stop the growth of these cancer cells. Before menopause, the ovaries make too much aromatase for the drug to block, so it is mainly used in women past menopause.
Why it comes up at the gym
To stop steroids from turning into estrogen
Testosterone, a male hormone, is partly turned into estrogen by aromatase. When steroids raise estrogen, men can develop breast growth called 'gynecomastia'. So steroid users take these drugs alongside without a prescription to prevent it, and the World Anti-Doping Agency bans them at all times.
Hormone changes in men
Estrogen goes down, testosterone goes up
Estrogen also tells the brain to 'stop making male hormones'. So when an aromatase inhibitor lowers estrogen, a man's testosterone level rises. In a trial in older men with low testosterone, estrogen fell and testosterone rose about 1.6 times.
Side effects
Joint pain and broken bones
People taking aromatase inhibitors often have joint, bone and muscle pain and hot flashes. In a large trial in women past menopause with breast cancer, broken bones were about 1.3 times as common while taking the drug as with another breast cancer drug (tamoxifen). After stopping the drug, there was no difference.
When a man's estrogen drops too low
More body fat and less sex drive
In a trial that controlled male hormones and estrogen separately in healthy men, body fat went up when estrogen was low. Low male hormones and low estrogen both played a part in lower sex drive. The loss of muscle and strength came from low male hormones.
Common misconception
Estrogen is a female hormone, so do men not need it?
No. Men's bones need estrogen too. A man born without aromatase had plenty of male hormones, yet at age 24 his bones had only matured to the level of a 14-year-old, and he had osteoporosis, where bones become weak. Older men who took an aromatase inhibitor also had lower 'bone density' in the spine, a measure of how solid bones are, than those on a fake pill.
Summary
A breast cancer drug that blocks estrogen production
An aromatase inhibitor is a drug that blocks aromatase, the enzyme that turns male hormones into estrogen, and so lowers estrogen; it was made to treat breast cancer. Men need estrogen too, so when it drops too low, bones weaken. Related terms: estrogen, gynecomastia, SERM.