💉 Letrozole vs Clomid for Ovulation Induction: Pros, Cons, and Side Effects

If you have been told you need help ovulating — whether because of PCOS, irregular cycles, or unexplained infertility — you will likely be offered either letrozole (Femara) or clomiphene citrate (Clomid). Both are inexpensive oral pills taken for 5 days early in your cycle, both have been used for decades, and both stimulate your ovaries to grow a mature follicle.

But the evidence has shifted decisively in the last decade. For many patients, especially those with PCOS, letrozole is now the preferred first-line agent. Here is what the data actually shows.

How Each Drug Works

Clomiphene citrate (Clomid)

Clomid is a selective estrogen receptor modulator (SERM). It blocks estrogen receptors in the hypothalamus, tricking the brain into thinking estrogen levels are low. The brain responds by releasing more FSH, which stimulates follicle growth.

Dosing: typically 50 to 150 mg daily for 5 days, starting on cycle day 3, 4, or 5.

Letrozole (Femara)

Letrozole is an aromatase inhibitor — originally developed to treat breast cancer. It blocks the enzyme that converts androgens into estrogens, temporarily lowering estrogen levels. The brain again responds by releasing more FSH. However, the mechanism is shorter-acting and does not block estrogen receptors elsewhere in the body.

Dosing: typically 2.5 to 7.5 mg daily for 5 days, starting on cycle day 3, 4, or 5.

Letrozole is used off-label for ovulation induction in many jurisdictions but is now the recommended first-line agent in major guidelines.

PCOS: The PPCOS II Trial Changed Everything

The pivotal PPCOS II trial (Pregnancy in Polycystic Ovary Syndrome II), published in NEJM in 2014, randomized 750 women with PCOS to letrozole or clomiphene.

Results:

• Live birth rate: 27.5 percent with letrozole vs 19.1 percent with clomiphene
• Ovulation rate: 61.7 percent with letrozole vs 48.3 percent with clomiphene
• Twin pregnancy rate: similar (about 3 percent each)

Letrozole won decisively. Major guidelines (ASRM, ESHRE, the international PCOS guideline) now recommend letrozole as first-line ovulation induction for PCOS.

Unexplained Infertility: The AMIGOS Trial

For unexplained infertility with IUI, the AMIGOS trial (NEJM 2015) showed similar pregnancy rates between letrozole and clomiphene, with lower multiples and better endometrial effects on letrozole.

Side Effects: Letrozole Generally Wins

Clomid side effects

• Hot flashes: very common (about 30 to 50 percent)
• Mood swings: common, sometimes significant
• Visual disturbances: blurred vision, light sensitivity — uncommon but require stopping the drug if they occur
• Thinning of the endometrium: clomid blocks estrogen receptors in the uterus too, sometimes making the lining too thin for implantation
• Thickening of cervical mucus: can interfere with sperm transport
• Ovarian cysts: occasional

Letrozole side effects

• Hot flashes: less common than clomid
• Fatigue
• Mild headaches
• Bone pain (rare at fertility doses)
• Endometrial thickness: generally preserved, often thicker than with clomid
• Cervical mucus: usually preserved

Overall, patients tolerate letrozole better and report fewer disruptive symptoms.

Birth Defect Concerns: Resolved

Early reports from a single 2005 study raised concerns about birth defects with letrozole. Subsequent large studies have shown no increased risk of birth defects with letrozole compared to clomiphene or natural conception. The original concern has been addressed by the evidence, and major guidelines no longer flag birth defect risk as a barrier to letrozole use.

Multiple Pregnancy Risk

Both drugs increase twin pregnancy rates above baseline. The risk:

• Letrozole: roughly 3 to 5 percent twin rate
• Clomid: roughly 5 to 8 percent twin rate

Triplet pregnancies are rare with either oral agent (much more common with injectable gonadotropins).

When Clomid Might Still Be Used

• Patient and clinician familiarity, in jurisdictions where letrozole is harder to access
• Specific insurance coverage limitations
• Patient preference based on prior experience
• Some specific clinical scenarios

The practical default for most patients in 2026 is letrozole.

Monitoring During an Ovulation Induction Cycle

Whether on letrozole or clomid, monitoring typically includes:

• Baseline ultrasound on cycle day 2 or 3 to confirm no cysts
• Mid-cycle ultrasound to count developing follicles and measure endometrial thickness
• Possibly bloodwork (estradiol)
• Trigger shot if used, or ovulation prediction
• Timed intercourse or IUI
• Pregnancy test 2 weeks after ovulation

When to Switch Drugs

If you do not ovulate on a given dose, the dose is typically increased. If you fail to ovulate at maximum dose, switching agents (clomid to letrozole, or vice versa) is a reasonable next step. After 3 to 6 ovulatory cycles without pregnancy, moving to IUI with stimulation or stepping up to IVF is usually discussed.

Practical Tips

Take the pills at the same time each day. Track symptoms in a journal. Use ovulation predictor kits if your clinic does not do mid-cycle ultrasound. Plan timed intercourse or IUI around predicted ovulation.

Confirm dosing, timing, and monitoring with your reproductive endocrinologist or family physician. To find clinicians offering ovulation induction, see the Fertility Link Navigator.