Follistim is an injectable fertility medication that stimulates your ovaries to grow multiple egg-containing follicles at once, instead of the single follicle your body typically matures each month. It contains a lab-made version of follicle-stimulating hormone (FSH), the same hormone your pituitary gland naturally produces to drive egg development. In IVF, the goal is to retrieve as many mature eggs as possible in a single cycle, and Follistim is one of the primary drugs used to make that happen.
How Follistim Works in Your Body
During a normal menstrual cycle, your brain releases FSH to coax one dominant follicle to maturity. The other follicles that started growing alongside it essentially get outcompeted and fade away. Follistim overrides this process by flooding your ovaries with much higher levels of FSH than your body would produce on its own, giving multiple follicles the hormonal support they need to keep growing simultaneously.
At the cellular level, FSH acts on the granulosa cells that surround each egg inside a follicle. These cells respond by producing estradiol, a form of estrogen that helps the follicle expand and the egg inside it mature. Your fertility team tracks your rising estradiol levels through blood draws throughout your stimulation cycle, using them alongside ultrasound measurements to gauge how your follicles are responding.
What a Typical Stimulation Cycle Looks Like
Most IVF protocols start Follistim injections early in your cycle. A common starting dose is 150 to 225 international units (IU) per day, given for at least the first four days before any adjustments. Your clinic will monitor you with ultrasounds and blood work, then raise or lower your dose based on how your follicles are responding. For women who respond slowly, the dose can go as high as 600 IU per day. For strong responders, the dose may be reduced or paused to avoid overstimulation.
The stimulation phase typically lasts 7 to 12 days. You’ll inject Follistim daily during this window, with monitoring appointments every two to three days. Once ultrasound shows a sufficient number of follicles measuring 12 to 19 millimeters in diameter, your doctor stops the Follistim and administers a “trigger shot” of hCG to induce final egg maturation. Egg retrieval then happens roughly 36 hours later.
Giving Yourself the Injection
Follistim comes in a prefilled cartridge that loads into a pen-style injector, similar to an insulin pen. The needle is a thin 29-gauge, which is about as fine as injection needles get. You inject it subcutaneously, meaning just under the skin rather than into muscle. The two recommended sites are just below your belly button or the upper outer area of your thigh.
Most people find the injection itself relatively painless, though mild stinging or redness at the site is normal. Your clinic will walk you through your first injection, and the pen has a dial that lets you set the exact dose your doctor prescribed. Each cartridge holds a set number of units, and you may need to swap in a new cartridge partway through your cycle depending on your dose.
Side Effects and Ovarian Hyperstimulation
The most common side effects during Follistim use are bloating, abdominal discomfort, mild nausea, and tenderness near the ovaries. These are expected consequences of growing multiple follicles, and they tend to increase as the stimulation cycle progresses. Some women also experience headaches, fatigue, or mood changes.
The more serious concern is ovarian hyperstimulation syndrome (OHSS), which occurs in roughly 1% to 5% of IVF cycles. OHSS happens when the ovaries overreact to hormonal stimulation, causing fluid to leak from swollen follicles into the abdomen. Mild cases cause bloating and discomfort that resolves on its own. Moderate cases involve visible fluid buildup in the abdomen on ultrasound. Severe cases, which are uncommon, can cause significant abdominal swelling, difficulty breathing, rapid weight gain of more than a kilogram in 24 hours, and in rare instances, blood clots or kidney problems.
Your risk for OHSS is higher if you have a high baseline follicle count (above 24), if more than 17 follicles grow past 10 millimeters during stimulation, or if your estradiol level climbs above 3,500 pg/mL by the time of the trigger shot. Your clinic monitors for these thresholds and can adjust your protocol accordingly, whether that means lowering your Follistim dose, switching to a different trigger medication, or freezing all embryos and transferring in a later cycle to let your ovaries settle down.
How Follistim Compares to Gonal-F
Follistim (follitropin beta) and Gonal-F (follitropin alpha) are the two main recombinant FSH products used in IVF. Both are lab-made versions of FSH with slight differences in their carbohydrate side chains, but these structural differences don’t translate into meaningful clinical differences. A large retrospective study comparing the two found virtually identical cumulative live birth rates: 52.8% for Follistim versus 55.7% for Gonal-F, a gap that was not statistically significant. Regression analysis adjusting for other variables confirmed that the type of FSH product had no meaningful association with outcomes.
The practical differences come down to cost and availability. Pricing varies by pharmacy, insurance coverage, and region, so your clinic may recommend one over the other based on what’s most accessible or affordable for you. If your doctor prescribes one and your pharmacy stocks the other, the clinical evidence suggests either will work equally well.
What Determines Your Response to Follistim
Not everyone responds to the same dose in the same way. Your starting dose is chosen based on factors like your age, body weight, baseline follicle count, and AMH level (a blood marker that reflects your ovarian reserve). Younger patients with high ovarian reserve often need lower doses, while older patients or those with diminished reserve may need higher doses for a longer stimulation period.
Your clinic adjusts the dose in real time based on monitoring. If your follicles are growing too slowly after the first four days, the dose goes up. If too many follicles are developing or your estradiol is rising too steeply, the dose comes down. This is why the monitoring appointments matter so much: Follistim is a powerful hormone, and the right dose is the one that produces enough mature eggs without pushing your ovaries into dangerous territory. The number of mature eggs retrieved can vary widely, from just a few to 20 or more, depending on your individual biology and how the cycle is managed.