Can Letrozole Cause the Formation of Ovarian Cysts?

Letrozole can cause ovarian cysts, but the risk during standard fertility treatment is low and comparable to or even lower than alternatives like clomiphene citrate. Cysts that do form are usually functional, meaning they arise from the normal follicular activity the drug is designed to stimulate, and they almost always resolve on their own. The story gets more complicated in other treatment contexts, though, where cyst formation has been frequent enough to derail entire treatment plans.

Why Letrozole Can Lead to Cyst Formation

Letrozole works by blocking aromatase, the enzyme that converts androgens into estrogen. When estrogen levels drop, the brain’s pituitary gland responds by releasing more follicle-stimulating hormone (FSH). That FSH surge is what drives follicle growth in the ovaries, which is exactly the point when the drug is used for ovulation induction. But follicles are, in structural terms, fluid-filled sacs. A follicle that grows large but does not rupture to release an egg, or one that fills with fluid after ovulation, becomes what clinicians call a functional cyst.

This is not a defect unique to letrozole. Any medication or process that stimulates follicular development carries the possibility of cyst formation. The follicle has to grow before it can ovulate, and growth sometimes overshoots or stalls. Letrozole’s mechanism makes this somewhat less likely than with older ovulation-induction drugs because the estrogen-lowering effect is temporary and relatively targeted. The drug has a short half-life, so estrogen levels bounce back quickly once treatment stops, and the ovaries are not stimulated as aggressively as they are with injectable gonadotropins.

How Often Do Cysts Form During Fertility Treatment?

In randomized trials of letrozole for ovulation induction, cyst formation shows up as an uncommon side effect. A double-blind trial comparing letrozole to clomiphene citrate in women with polycystic ovary syndrome (PCOS) recorded cyst formation in three participants on letrozole and three on clomiphene, out of groups of roughly equal size. One serious hemorrhagic cyst occurred in each treatment arm, and both resolved without lasting consequences.¹ These numbers suggest that while cysts can happen, they are not dramatically more common with letrozole than with the main alternative fertility drug.

A separate randomized trial in clomiphene-resistant PCOS patients found that the incidence of ovarian cysts in the letrozole group was actually lower than in the group receiving human menopausal gonadotropin (HMG), with the difference reaching statistical significance.² This is consistent with what fertility specialists generally observe: letrozole tends to produce gentler ovarian stimulation than gonadotropin injections, which means fewer large follicles, fewer cysts, and a lower risk of ovarian hyperstimulation syndrome.

Letrozole Compared to Clomiphene for Cyst Risk

The comparison between letrozole and clomiphene citrate matters because clomiphene was the first-line ovulation-induction drug for decades, and many patients switching to letrozole want to know whether they are trading one set of risks for another. The evidence suggests the cyst risk is roughly equivalent. In the double-blind trial mentioned above, the raw numbers of cyst-related adverse events were identical between the two arms.¹

Where letrozole does seem to outperform clomiphene is in a related problem called luteinized unruptured follicle (LUF) syndrome. This is when a follicle develops and the body goes through the hormonal motions of ovulation, including progesterone production, but the egg never actually releases. The follicle stays intact and can persist as a cyst. A randomized trial comparing letrozole-HMG to clomiphene-HMG regimens found significantly fewer cases of complete LUF in the letrozole group, along with a higher pregnancy rate.³ That lower LUF rate is clinically meaningful because it means fewer persistent cysts lingering into the next cycle and potentially delaying treatment.

The Endometriosis Exception

If you have read this far and think cyst formation on letrozole sounds uncommon, there is a significant caveat. When letrozole has been tested for treating endometriosis rather than inducing ovulation, cyst formation has been far more problematic.

A study of women with stage IV endometriosis who were given letrozole alongside a desogestrel-only contraceptive pill found that every single participant developed ovarian cysts. None of the women completed the planned six-month course of treatment. The median time before treatment had to be stopped was about 84 days, with a range of roughly 56 to 112 days.⁴ The women did report meaningful improvements in pain while on the drug, but symptoms came back within three months of stopping.

The reason for this dramatic difference probably lies in the treatment context. In ovulation induction, letrozole is given for a short window, typically five days early in the menstrual cycle. The goal is a brief FSH surge. In the endometriosis protocol, the drug was taken continuously over months, and the combination with a progestogen-only pill created a hormonal environment where follicular cysts could grow unchecked. This serves as a useful reminder that the same drug can behave very differently depending on the dose schedule, duration, and what it is combined with.

Letrozole as Protection Against Ovarian Hyperstimulation

Somewhat counterintuitively, letrozole is also used to prevent the most dangerous form of ovarian over-response: ovarian hyperstimulation syndrome (OHSS). OHSS occurs when the ovaries swell dramatically, sometimes leading to fluid buildup in the abdomen, blood clots, and in severe cases, kidney failure. It is most common in IVF cycles using gonadotropin injections, and women with PCOS are at elevated risk.

Research has explored giving letrozole during the luteal phase (after egg retrieval in IVF) to bring estrogen levels down quickly and reduce the severity of OHSS. A study of high-risk patients found that a dose of 7.5 mg daily for five days after egg retrieval significantly reduced the incidence of OHSS, while lower doses of 2.5 mg and 5 mg showed only a modest trend toward benefit.⁵ Another randomized trial found that adding letrozole to a GnRH antagonist protocol in PCOS patients undergoing IVF produced only one moderate OHSS case in the letrozole group compared with nine in the control group.⁶

Not every study has found a statistically significant benefit, though. A retrospective study comparing letrozole-treated and untreated women at risk of OHSS during IVF found a lower incidence in the letrozole group (about 2.6% versus nearly 7.8%), but the difference did not reach significance, likely because of the small sample sizes involved.⁷ Another retrospective analysis showed a similar trend that fell short of significance.⁸ The overall picture is encouraging but not airtight, and the optimal dose and timing are still being worked out.

Monitoring and What to Expect If a Cyst Appears

Most fertility clinics perform at least one ultrasound during a letrozole cycle, usually around the time ovulation is expected. One of the specific reasons for this monitoring is to check for ovarian cysts. A textbook chapter on reproductive medicine notes that ultrasound during letrozole treatment is “highly recommended for safety when ruling out possible underlying pregnancy and ovarian cysts.”⁹ The title of that chapter argues that monitoring is not strictly required for uncomplicated letrozole cycles, which gives you some sense of how benign the drug’s profile is considered, but the chapter itself still recommends imaging in certain situations.

If a cyst is found, the usual approach is to wait. In the double-blind trial discussed earlier, both hemorrhagic cysts that occurred resolved spontaneously without surgery.¹ Most functional cysts shrink within one to two menstrual cycles. Your doctor may advise skipping a treatment cycle to give the cyst time to resolve before restarting medication.

Rarely, a cyst can cause sudden sharp pain if it ruptures or twists the ovary (a condition called ovarian torsion), which is a medical emergency. But torsion is uncommon with the modest follicular growth that letrozole typically produces. It is more of a concern with aggressive gonadotropin protocols that generate multiple large follicles.

Do Pre-Existing Cysts Affect Your Treatment?

A question that comes up frequently in fertility clinics is whether you should delay treatment if a cyst is already present at your baseline ultrasound. Many clinics have a long-standing policy of canceling cycles when a cyst is seen, reasoning that the cyst could grow further or interfere with new follicular development. The evidence suggests this concern is somewhat overblown.

A study of ovulation-induction and intrauterine insemination cycles found that clinical pregnancy rates were nominally lower in women who had a simple cyst at baseline compared to those who did not (about 9% versus 15%), but the difference was not statistically significant. After adjusting for age and body mass index, there was still no meaningful difference in pregnancy rates between the two groups.¹⁰ This suggests that simple functional cysts found before starting letrozole do not necessarily doom the cycle, though individual circumstances always matter. A large or complex cyst is a different clinical decision than a small simple one.

Letrozole Used to Treat Ovarian Cysts in Children

In a twist that underscores how context-dependent drug effects are, letrozole is sometimes prescribed specifically to manage ovarian cysts in a pediatric setting. Young girls who develop autonomous ovarian cysts can experience peripheral precocious puberty, where the cyst produces estrogen and triggers early breast development or menstruation. Letrozole, by blocking aromatase, can suppress the estrogen output from these cysts and halt the precocious puberty symptoms while the cyst resolves naturally.

A case report described a nine-year-old girl who developed a functioning ovarian cyst causing premature menstruation. She was started on oral letrozole, had no further menstrual episodes, and the cyst resolved spontaneously after three months.¹¹ A larger review of girls with peripheral precocious puberty from autonomous ovarian cysts found that most cysts spontaneously disappeared, with a median time to regression of about six months.¹² In this context, letrozole is not causing cysts but rather managing the symptoms they produce, buying time until the body resolves the problem on its own.

When Cyst Risk Is Higher Than Average

Not everyone faces the same odds. Several factors can tilt the probability of developing a cyst during letrozole treatment:

PCOS: Women with polycystic ovary syndrome already have ovaries filled with small follicles. Stimulation can sometimes push one or more of these follicles into cyst territory. Paradoxically, letrozole is a first-line treatment for PCOS-related anovulation precisely because it stimulates these ovaries more gently than alternatives.
Extended or continuous dosing: The endometriosis study showed that months of uninterrupted letrozole produced cysts in every participant. The standard five-day protocol used in fertility treatment carries much less risk.
Combination protocols: Adding gonadotropins to letrozole increases the total ovarian stimulation. The cyst risk in combined protocols is higher than with letrozole alone, though still often lower than with gonadotropins by themselves.
Individual ovarian sensitivity: Some women simply respond more vigorously to any form of ovarian stimulation. Prior history of cysts or strong response to low-dose medications is a signal your doctor will watch for.

Your prescribing clinician should be calibrating the protocol based on these factors. If you have a history of cysts or are on a protocol that combines letrozole with other stimulation medications, expect closer monitoring with additional ultrasound visits.

Cysts in the IVF Context

When letrozole is used as part of an IVF cycle rather than a simpler ovulation-induction protocol, the dynamics shift. In IVF, the goal is to grow multiple follicles simultaneously, and letrozole is sometimes added to the stimulation protocol to reduce the total gonadotropin dose needed or to lower estrogen levels in women at risk of OHSS. The cyst concern in this setting is less about letrozole-specific cysts and more about the overall intensity of stimulation.

Letrozole has also been studied in a specialized context: fertility preservation for women with estrogen-sensitive cancers, such as early-stage serous borderline ovarian tumors. A case report described successful egg freezing in young women with these tumors who underwent ovarian stimulation with simultaneous letrozole. The researchers specifically noted that there were no changes in the appearance of pre-existing ovarian cysts during treatment.¹³ While this is a small case report and far from definitive, it does suggest that carefully managed letrozole protocols can be safe even in patients who already have cystic ovarian pathology.

For women undergoing IVF who are worried about OHSS, the evidence on letrozole as a protective measure is encouraging. One randomized trial in PCOS patients found that adding letrozole to the stimulation protocol cut moderate OHSS cases from nine to one.⁶ The mechanism is straightforward: by lowering estrogen, letrozole reduces the vascular permeability and fluid shifts that drive OHSS. This makes letrozole one of the tools in the OHSS prevention toolkit, alongside trigger modification, freeze-all strategies, and cabergoline.

What to Tell Your Doctor

If you are starting letrozole and are concerned about cysts, it helps to frame the conversation around your specific situation. Let your doctor know if you have a history of ovarian cysts, if you have been diagnosed with PCOS, if you have endometriosis, or if you have had previous reactions to ovarian-stimulating medications. All of these change the risk calculus.

During treatment, mild pelvic heaviness or a dull ache on one side is common and usually just means a follicle is growing as intended. Sharp, sudden, or severe pain is a reason to call your clinic immediately. Most women on standard letrozole cycles will never know whether they briefly had a small cyst, because functional cysts at this scale often come and go silently between ultrasounds. The important thing is to keep your scheduled monitoring appointments so that anything unexpected can be caught early, and to understand that if a cyst does appear, the most likely outcome is that it resolves on its own within a cycle or two.