Letrozole_Dosage, benefits & legal status
Also known as Femara, letrozol, LETRO
The stronger aromatase inhibitor, and the one with the clearest bone cost.
Common vial size is what vendors typically sell, not a recommendation. Enter your own vial in the calculator.
What is Letrozole?
Plain language first. The technical label stays, but it never stands alone.Overview
It shuts down estrogen production more completely than anastrozole does.
Letrozole is a non-steroidal aromatase inhibitor that competitively binds the heme group of the aromatase enzyme, cutting estrogen production across every tissue that makes it.
What it's used for
A breast cancer drug used on the research market when anastrozole is not holding estrogen down far enough.
How does Letrozole work?
What it does in the body, and where.Letrozole and anastrozole do the same job. Letrozole does more of it.
- 01It binds the heme group of aromataseThis is a competitive block on the enzyme that converts androgens into estrogen.
- 02Estrogen production falls across every tissueThe label notes it is highly specific, with no effect on adrenal steroid production, so cortisol and aldosterone are left alone.
- 03Bone pays for itThe bone substudy measured a median 4.1 percent fall in lumbar spine density at 24 months, against a small rise on tamoxifen.
- 04Nothing about this is gentleThe head-to-head trial data shows more fractures on letrozole and more clotting and endometrial events on tamoxifen. Each drug has its own cost.
A schematic of the compound's mechanism, not a diagnostic image.
What else is it used for?
Each use below carries its own evidence level. Approved means regulators have cleared it for that purpose. Investigational means it is in trials. Early research means it is being studied and is not established.
- Hormone receptor-positive early and advanced breast cancer in postmenopausal womenFDA-approved
- Ovulation induction, off-label and widely used in fertility practiceClinical use
- Controlling estrogen during an anabolic cycleResearch only
How much, and do you cycle it?
The range, and whether you take breaks.2.5 mg per day, daily. Continuous, for about five years in the trialsFDA label
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.
How long does it stay in the body?
The half-life, and where the figure comes from.About 2 daysFDA label
Commonly reported as roughly two days, with steady state reached after two to six weeks of daily dosing. We have not read the exact figure off the current label, so the range shown is approximate and flagged.
Not yet traced to a primary source.
You can compare this against the whole library to see where it sits.
What does it cost?
Typical dose and cost, in plain numbers.Letrozole is not injected
It is taken by mouth (oral). One 2.5 mg tablet once a day, with or without food, in every approved indication. Splitting tablets to a fraction of this, which is common practice alongside testosterone, has no label and no trial behind it.
We track Letrozole at 1 vendor, and publishes its price.
1 listing, last read Aug 22, 2026. Shipping and card fees are not included.
How do you store it?
Before mixing, and after.Room temperature. Cool, dry, out of light.
Letrozole is taken as a pill or capsule. Keep it in its original container at room temperature, away from heat, humidity, and direct sun. No fridge or mixing needed. Check the expiration date on the bottle.
What are the side effects and cautions?
Known cautions and warnings for this compound.Who should avoid it
- 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
Stop and get help
- A bone fracture from a minor fall or knock
- Severe or sudden joint pain, or tendon pain, since tendonitis and rupture are reported
- Dizziness or drowsiness severe enough to affect driving
- Signs of an allergic reaction such as face or throat swelling, hives, or trouble breathing
Common effects
- Hot flushes
- Joint pain
- Fatigue and weakness
- Dizziness and drowsiness
- Raised cholesterol
- Falling bone density over months
Where this page says nothing is established, that means nobody has studied it, not that a compound is safe.
Is Letrozole approved?
And where the numbers on this page come from.Yes. Letrozole is FDA approved and available on prescription in the United States.
- Approval statusFDA-approved
- RouteBy mouth (oral)
- CheckedAugust 2026
What has happened recently
First approved in the US in 1997, for hormone receptor-positive breast cancer in postmenopausal women. The label contraindicates it in women of premenopausal hormonal status and states that clinical benefit in premenopausal women has not been demonstrated. There is no approved use in men. It carries warnings on bone mineral density and on total cholesterol, and product monographs also note tendonitis, tenosynovitis and rare tendon rupture across this drug class. Prohibited in sport at all times by WADA under section S4.
Banned in drug-tested sport. Most research peptides are prohibited under WADA's S0 category, which covers any substance with no approval from any regulatory authority for human use. That means a compound can be banned without ever being named on the prohibited list. Check with your own anti-doping authority rather than assuming absence from the list means permitted.
You can compare this against the whole library, or read how to check a seller.
- DoseFrom the drug label. The manufacturer tested this range and a regulator reviewed it.
- Vial sizeFrom what vendors typically list; not a recommendation.
- Sources
- Reviewed24 August 2026
What else is like Letrozole?
Others in Sexual and hormonal, side by side.If you're weighing Letrozole, it's worth reading Toremifene, Cabergoline, and PT-141 (Bremelanotide) in the same class.
The stronger aromatase inhibitor, and the one with the clearest bone cost.
- Dose
- 2.5 mg per day
- How often
- Daily
A SERM with a boxed warning for a heart rhythm that can kill.
- Dose
- 60 mg per day
- How often
- Daily
A prolactin drug whose label asks for an echocardiogram before the first dose.
- Dose
- 0.25 up to 1 mg twice a week
- How often
- Twice a week
An approved shot that boosts sexual desire through the brain.
- Dose
- 1.75 mg per dose
- How often
- As needed, before activity
10What these words mean
Show glossary +
What these words mean
- 503A and 503B
- The two kinds of compounding pharmacy in US law. A 503A pharmacy prepares medicine for one named patient. A 503B outsourcing facility makes larger batches under stricter manufacturing rules.
- Agonist
- Something that switches a receptor on. The opposite is an antagonist. It blocks the receptor instead.
- Amino acid
- The building blocks that link together in a chain to make a peptide or a protein. The order they sit in is what makes one peptide different from another.
- Amylin
- A hormone released alongside insulin that slows the stomach emptying and reduces appetite. Several weight loss compounds copy it.
- Analog
- A compound built to copy a natural one with small deliberate changes, usually so it lasts longer or holds onto its target more tightly.
- Animal study
- Research done in rats, mice or other animals. Most findings do not repeat in people, so an animal result is a reason to keep looking. It is not a reason to act.
- Antibody
- A large immune protein, far bigger than a peptide, engineered to lock onto one specific target. These are made by drug companies and are not sold as research powders.
- Bacteriostatic water
- Sterile water with a tiny amount of preservative, used to mix powdered peptides.
- Banned in drug-tested sport
- Prohibited by the World Anti-Doping Agency (WADA); using it can disqualify a competing athlete.
- Bioavailability
- How much of a dose actually reaches your bloodstream. Most peptides survive stomach acid badly, so most of them are injected instead of swallowed.
- Bioregulator
- Very short peptides from a Russian research tradition, usually two to four amino acids long, sold for supporting a specific organ. Human evidence is thin and most of it is published in Russian.
- Blend
- One vial holding more than one compound, mixed together by the seller. Working out a dose means working out each ingredient separately, because they are rarely present in equal amounts.
- Boxed warning
- The most serious warning the FDA puts on a drug label, printed inside a black border. It flags a risk considered severe enough to sit above everything else on the label.
- Bulks list
- The FDA's list of ingredients that compounding pharmacies are allowed to use. A substance can sit under review on it for years without anything being decided either way.
- Certificate of analysis (COA)
- A lab report on a batch of product. It only means something if it names the batch you actually received, comes from an independent lab, and can be checked with that lab.
- Clinical trial
- A study that gives a treatment to people under a written plan and measures what happens. It is the only kind of evidence that can show something works in humans.
- Compounding
- A licensed pharmacy preparing a medicine to order. Being allowed to compound something is not the same as the FDA approving it, and the two get mixed up constantly.
- Concentration
- How strong the mixed peptide is, in milligrams per milliliter (mg/mL). More water makes it weaker; less water makes it stronger.
- Cycle
- A stretch of time using a compound followed by a break. Most cycling patterns come from community practice. Very few have been tested.
- DAC
- Short for Drug Affinity Complex. An attachment added to a peptide so it grabs onto a blood protein and stays in the body for days instead of minutes.
- Dead space
- The small amount of liquid left sitting inside the needle and hub after you inject. On very small doses it can be a real share of what you meant to take.
- Dilute
- Adding more liquid to make a mix weaker. The amount of peptide in the vial does not change. It is just spread through more water, so each draw is bigger.
- Endpoint
- The result a trial said in advance that it would measure. A study can be reported as positive and still have missed the endpoint it set out to hit.
- FDA approved
- Reviewed by the FDA and cleared for a specific use, in a specific form, at a specific dose. Approval for one use is not approval for another, and approval in another country is not approval here.
- Fragment
- A short piece cut out of a larger natural protein. A fragment can behave very differently from the whole protein it came from, so evidence about one does not transfer to the other.
- Ghrelin
- The hunger hormone. Some growth hormone compounds work by copying it, so they can make you noticeably hungrier.
- GHRH
- A natural signal telling the pituitary gland to release growth hormone. Several compounds copy it.
- GHRP
- A second, separate signal that also triggers growth hormone release, through a different door than GHRH. This is why the two types are often used together.
- GLP-1
- A natural gut hormone that lowers appetite and blood sugar. Drugs that copy it, like semaglutide, are used for weight loss.
- Growth hormone secretagogue
- A compound that nudges your body to release more of its own growth hormone, rather than injecting the hormone itself.
- Half-life
- How long a compound stays active in your body. A longer half-life means you inject less often.
- HPLC
- A lab method that separates out everything in a sample so the amounts can be measured. It shows how much of the main ingredient is there. It does not confirm that the main ingredient is the right molecule.
- In vitro
- Done in a dish or a test tube, on cells outside a living body. It is the earliest kind of evidence there is.
- Injection site
- The spot on the body where you inject. Moving it around gives the skin and fat underneath time to recover between doses.
- Intramuscular (IM)
- An injection into a muscle, using a longer needle.
- Intravenous (IV)
- An injection into a vein. It needs training and equipment and is not something to do at home.
- Investigational
- Still being tested in clinical trials and not yet approved or legally sold for general use.
- Lyophilized
- Freeze-dried into a powder so it keeps longer. That is the white cake or dust in the bottom of an unmixed vial.
- Mass spectrometry
- A lab method that weighs molecules to confirm what they are. HPLC answers how much is in the sample. This answers what it is.
- mcg and mg
- Units of dose. 1 milligram (mg) equals 1,000 micrograms (mcg).
- Myostatin
- A natural brake on muscle growth. Several compounds are built to block it, though human results have been far smaller than the animal work suggested.
- Off-label
- A doctor prescribing an approved drug for something other than its approved use. This is legal and common, and it is different from a compound having no approval at all.
- Orphan drug designation
- A status given to treatments for rare conditions to make them worth developing. It is an incentive, not an approval, and a compound can hold it while being approved for nothing.
- PCAC
- The FDA advisory committee that reviews compounding ingredients. It recommends. It does not approve, and the FDA does not have to follow what it says.
- Peer review
- Other scientists checking a paper before a journal publishes it. It filters out some bad work. It does not make a finding true.
- Peptide vs small molecule
- A peptide is a short chain of amino acids. A few items here (like MK-677 or methylene blue) are small molecules, not true peptides, but people use them the same way.
- Placebo
- A dummy treatment given for comparison. Without one it is very hard to tell a real effect apart from expectation and time passing.
- Purity
- The share of a sample that is the intended compound, usually given as a percentage. A high purity number on the wrong molecule is still the wrong molecule.
- Randomized
- Deciding by chance who gets which treatment, so the two groups are comparable. Studies that skip this are much easier to read the wrong way.
- Receptor
- A docking point on a cell that a compound attaches to. If a cell has no matching receptor, the compound does nothing to it.
- Reconstitution
- Mixing a powdered peptide with liquid (bacteriostatic water) so it can be drawn into a syringe.
- Research only
- No approved dose or use in people. Sold for research and not reviewed by the FDA for safety or quality.
- Stack
- Using more than one compound at the same time. Almost no combinations have been tested together in people, so a stack is usually built on guesswork.
- Sterile water
- Water with nothing added to stop bacteria growing. It works for a single dose, but a vial mixed with it should not be kept and used again.
- Subcutaneous
- An injection just under the skin, into the fat layer, usually with a short, thin insulin needle.
- Titrate
- Starting at a low dose and increasing it slowly over time so your body adjusts and side effects stay mild.
- Trial phases
- The stages of human testing. Phase 1 checks safety in a small group, phase 2 looks for an effect, and phase 3 tests it against a comparison in a large group.
- Units (on the syringe)
- The marks on an insulin syringe. 100 units equals 1 mL. They measure how much liquid you draw, not the dose in milligrams.
Guides for this compound
- How to Reconstitute a Peptide
Mix peptide powder with bacteriostatic water, step by step.
- mcg vs mg: The Peptide Dosing Mistake to Avoid
Your vial says mg and your protocol says mcg. Here is the conversion and how to check it.
Sources
- Femara prescribing information, DailyMed(2024)1997 initial US approval, premenopausal contraindication, bone and cholesterol warnings.
- Femara label PDF, FDA Drugs@FDA(2024)Bone substudy figures and the 96-month fracture comparison against tamoxifen.
Last reviewed 24 August 2026
Peptide Decoding is published by Decoded Sciences LLC. We take no payment from vendors for coverage, inclusion or ranking, and our affiliate relationships are disclosed in full.