Anastrozole and Letrozole are each a small molecule rather than a peptide.
| Comparison | Anastrozole Arimidex · ANA | Letrozole Femara · letrozol |
|---|---|---|
| How it works | How it works Anastrozole works upstream of the receptor, on supply rather than on signal. | How it works Letrozole and anastrozole do the same job. Letrozole does more of it. |
| What it is | An aromatase inhibitor approved for postmenopausal breast cancer. | The stronger aromatase inhibitor, and the one with the clearest bone cost. |
| Status | FDA approvedFDA-approved | FDA approvedFDA-approved |
| Typical dose | 1 mg per day | 2.5 mg per day |
| How often | Daily | Daily |
| Stays active | About 47 hours | About 2 days |
| Dose comes from | FDA drug label | FDA drug label |
| Cycled? | Continuous, for years in the trials FDA drug label | Continuous, for about five years in the trials FDA drug label |
| What it does | ||
| Drug class | Aromatase inhibitor (oral, not a peptide) | Aromatase inhibitor (oral, not a peptide) |
| Used for | A breast cancer drug used on the research market to hold estrogen down during a testosterone or anabolic cycle. | A breast cancer drug used on the research market when anastrozole is not holding estrogen down far enough. |
| Receptors hit | — | — |
| Numbers | ||
| Full dose line | 1 mg per day | 2.5 mg per day |
| Why that cycle | Given daily and without a break for the duration of treatment. The intermittent twice-weekly dosing used alongside testosterone is a community pattern built around symptom control rather than anything from a trial. | Median treatment duration in both the adjuvant and the extended adjuvant studies was five years, given daily throughout. The label states that the optimal duration is not known. There is no trial anywhere of the short intermittent courses used alongside a cycle. |
| Weight / effect | Not measured in a trial | Not measured in a trial |
| Chain length | Not disclosed | Not disclosed |
| Common vial | — | — |
| Before you start | ||
| Route | By mouth (oral) | By mouth (oral) |
| Do not use if | Pregnancy Existing osteoporosis or low bone density, without a plan to manage it Known hypersensitivity to anastrozole Pre-existing ischaemic heart disease, where the label notes more ischaemic events on anastrozole than on tamoxifen Banned in drug-tested sport | Premenopausal hormonal status, which the label states outright Pregnancy, or any possibility of pregnancy Existing osteoporosis or low bone density, without a plan to manage it Known hypersensitivity to letrozole Banned in drug-tested sport |
What people actually report
Anastrozole
- It lowers bone mineral density, and the effect showed up within twelve months in the trials.
- Cholesterol rises. In the ATAC trial 9 percent of women on anastrozole had raised cholesterol against 3.5 percent on tamoxifen.
- Joint pain and stiffness are the complaints that most often stop people taking it.
- Banned in drug-tested sport at all times.
Stop and get help if
- Chest pain, or new breathlessness on exertion
- A bone fracture from a minor fall or knock
Letrozole
- Bone loss is measurable and larger than with tamoxifen. At 24 months the median lumbar spine density fell 4.1 percent on letrozole against a 0.3 percent rise on tamoxifen.
- At 96 months of follow-up 14.7 percent of women on letrozole had a fracture event against 11.4 percent on tamoxifen.
- The label contraindicates it in anyone of premenopausal hormonal status.
- Banned in drug-tested sport at all times.
Stop and get help if
- A bone fracture from a minor fall or knock
- Severe or sudden joint pain, or tendon pain, since tendonitis and rupture are reported
Side effects are what users and trials report most often, not a complete list. Anything sudden, spreading, or breathing-related is an emergency regardless of which compound caused it.
People also compare
This comparison does not cover cost, long-term safety, or how you personally will respond. Effect figures come from separate trials in different populations and are not always directly comparable to each other. Nothing here is medical advice or a recommendation to use any of these. Talk to a clinician about your own situation.