OnePin Peptide Library · Hormonal
Aromasin (Exemestane)
Exemestane
Exemestane, marketed as Aromasin, is an orally active steroidal aromatase inhibitor (AI) developed by Pharmacia & Upjohn and approved by the FDA in 1999 for the treatment of estrogen-receptor-positive breast cancer in postmenopausal women. It is a 6-methylenandrosta-1,4-diene-3,17-dione derivative - structurally an androstenedione analog - that binds irreversibly to the aromatase enzyme and inactivates it, earning it the informal label of suicidal or suicide inhibitor within the AI class.
Take with food to optimize absorption (~40% higher AUC fed vs fasted). Dosing is typically 12.5mg every other day to 25mg daily depending on the aromatizable steroid dose. Start low - crashing estrogen causes joint pain, low libido, depression, and lipid damage. Confirm dose with a pre-cycle E2 baseline and a mid-cycle E2 (sensitive assay) check.
Start here. Every protocol below is ordered lowest-first. Begin at the smallest effective dose, hold about a week to assess tolerance, and step up only if needed.
How it works
Aromatase is the CYP19A1 enzyme that converts androgens (testosterone, androstenedione) into estrogens (estradiol, estrone). Exemestane is an androstenedione analog that binds the aromatase active site and is then converted by the enzyme itself into a reactive intermediate that covalently modifies and permanently inactivates the enzyme. Restoration of aromatase activity therefore requires de novo synthesis of new enzyme rather than simple displacement.
Aromatase is the CYP19A1 enzyme that converts androgens (testosterone, androstenedione) into estrogens (estradiol, estrone). Exemestane is an androstenedione analog that binds the aromatase active site and is then converted by the enzyme itself into a reactive intermediate that covalently modifies and permanently inactivates the enzyme. Restoration of aromatase activity therefore requires de novo synthesis of new enzyme rather than simple displacement.
In postmenopausal women, peripheral aromatization in fat, skin, and muscle is the dominant estrogen source; exemestane 25mg daily produces roughly 85-98% suppression of whole-body aromatization and plasma estradiol. In men on aromatizable steroid cycles, a similar suppression translates to tight estrogen control through a stable blood-level curve without the rebound spikes seen with reversible AIs when doses are missed.
Exemestane also has weak androgenic activity of its own - it is structurally related to androstenedione and retains mild AR-binding - which is why it does not fully crash HDL the way strong reversible AIs tend to, and why some users report modest anabolic feel during use. Endogenous testosterone typically rises modestly through reduced estrogen negative feedback at the HPT axis.
What to expect
First 3-7 days: steady-state tissue aromatase suppression builds as irreversibly inactivated enzyme accumulates. Estrogen-related water weight and blood pressure often begin to drop.
Weeks 2-4: estrogen stabilizes at the dose-dependent set point. Gynecomastia symptoms resolve in most users if dose is appropriate. This is the window to draw a sensitive E2 and adjust.
Beyond 4 weeks on a stable dose: estrogen remains steady. Bone density and lipids should be monitored on long runs. Post-cycle recovery of aromatase function takes 1-2 weeks.
Evidence
Interactions & stacking
What's safe to combine, and what belongs in a separate pin.
Keep separate
Side effects & safety
Commonly reported
- Joint pain and stiffness (dose-related, worsens with over-suppression)
- Reduced libido and erectile quality if E2 is over-suppressed
- Mild fatigue or flat mood with low E2
- Hot flashes (more common in women; occurs in men with crashed E2)
- Mild lipid shift (less severe than non-steroidal AIs)
Less common & notes
- Dry skin and eyes, insomnia, headache, decreased bone mineral density on long-term use, mild elevation of liver enzymes, hair thinning.
- Rare but reportable: severe depression with over-suppression, significant osteopenia or fractures on multi-year adjuvant use, hypersensitivity reactions.
- Crashing estrogen below ~15 pg/mL for extended periods is the primary cause of symptomatic side effects in men on cycle.
Pharmacokinetics
Storage. Store at controlled room temperature (20-25°C / 68-77°F). Protect from moisture and light. Keep in original container. Shelf life typically 3 years from manufacture date. Keep out of reach of children.
Regulatory status
Prescription medication (Rx) in the United States. FDA-approved brand: Aromasin (Pfizer). Generic exemestane widely available. Not a controlled substance. Off-label use in men for estrogen control during AAS cycles or TRT is common but legally requires a valid prescription. Internationally classified as a prescription drug in most jurisdictions.
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