Three aromatase inhibitors are commonly used: anastrozole (Arimidex), exemestane (Aromasin) and letrozole (Femara). They differ in potency and in one structural respect that matters — exemestane binds irreversibly, the other two do not. Letrozole is the strongest and is usually the wrong choice.
What an aromatase inhibitor does
Aromatase is the enzyme that converts testosterone to oestradiol. An AI blocks it, so less oestrogen is produced and blood oestradiol falls. That is genuinely different from a SERM like tamoxifen, which leaves oestradiol alone and blocks the receptor.
Because an AI lowers circulating oestrogen, it addresses the things driven by oestrogen levels: water retention, blood pressure from that retention, and the oestrogenic contribution to gynecomastia risk.
The three compared
| Anastrozole | Exemestane | Letrozole | |
|---|---|---|---|
| Also known as | Arimidex | Aromasin | Femara |
| Binding | Reversible | Irreversible | Reversible |
| Relative potency | Moderate | Moderate | Strongest |
| Rebound on stopping | Possible | Minimal | Possible |
| Effect on lipids | Negative | More neutral | Negative |
| Usual role | On-cycle control | On-cycle control | Rescue only |
Why irreversible binding matters
Exemestane is a suicide inhibitor — it binds to the aromatase enzyme permanently, and the body must synthesise new enzyme to restore capacity. Anastrozole and letrozole bind reversibly and let go when levels fall.
The practical consequence is oestrogen rebound. Stop a reversible AI and the enzyme that has been accumulating becomes available at once, which can spike oestradiol. Exemestane does not produce that effect to the same degree, which is why it is often preferred at the end of a cycle heading into PCT.
Exemestane also has a more neutral effect on lipids, which matters because AIs damage HDL and you are usually running one alongside compounds already doing that.
Why letrozole is usually wrong
Letrozole is the most potent of the three, and that is the problem rather than the selling point. It can suppress oestradiol severely, and crashed oestrogen is its own clinical problem:
- Joint pain and dryness — oestrogen contributes to joint lubrication and collagen
- Complete loss of libido, often worse than the low-testosterone version
- Lipid damage beyond what the anabolic alone causes
- Low mood and lethargy
- Impaired bone mineral density over time
Letrozole has a legitimate role in reversing established gynecomastia under supervision. As routine on-cycle control it overshoots badly.
The mistake almost everyone makes
Dosing an AI without measuring oestradiol. Aromatisation varies enormously between people and between compounds — the same dose that leaves one person at a sensible level crashes another.
Oestrogen is not the enemy. It is necessary for libido, joint health, lipids, bone and mood. The goal is a normal range, not zero. People who feel terrible on cycle despite everything going right are very often crashed rather than high.
The only way to know is a sensitive oestradiol test alongside total testosterone. Dose from the number, not from a forum protocol.
What we carry
Our anastrozole (56 lines), exemestane (43) and letrozole (45) categories carry all three, under both generic and trade names — Arimidex, Aromasin, Femara, plus brand variants like Arimimed, Aromamed and Femamed.
Frequently asked questions
Which aromatase inhibitor should I use?
Anastrozole or exemestane for routine control, with exemestane preferred heading into PCT because of the rebound and lipid differences. Letrozole is a rescue drug, not a default.
Do I need an AI on every cycle?
Only with aromatising compounds, and only if oestradiol is actually elevated. Non-aromatising compounds like masteron or stanozolol do not need one.
What does crashed oestrogen feel like?
Aching joints, no libido, flat mood, lethargy — often mistaken for the cycle not working. A sensitive oestradiol test distinguishes it from the opposite problem in one measurement.