A protocol name such as "antagonist" or "long" tells you the order of your medicines. It says which shot starts first, when a second medicine joins it, and how long the plan takes. Your doctor recommends a protocol based on your test results and any past treatment. When you're offered one, ask about its expected benefits, risks, cost and alternatives.
What is an IVF protocol?
In IVF, hormone shots help several eggs grow at once. The protocol is the recipe for that part of treatment.
The antagonist and long protocols use two kinds of medicine:
- Stimulation shots (gonadotropins). These help the follicles grow. A follicle is the small fluid sac that holds an egg.
- A medicine to stop early ovulation. Without it, your body might release the eggs before retrieval. GnRH agonists and GnRH antagonists both do this job, in different ways.
There is no evidence that one stimulation shot works better than another.
At the end of stimulation, your clinic gives you a trigger medicine plan and an exact time to take it. Eggs are usually collected 34 to 36 hours later, which is why that time matters. Read more in IVF stimulation and monitoring.
IVF protocols compared
| Protocol | How it starts | Shots | What to know |
|---|---|---|---|
| Antagonist | Stimulation shots first, antagonist added later | Fewer days of shots | Recommended for most people in the 2025 ESHRE guideline; lower OHSS risk |
| Long agonist | Agonist starts in the cycle before | Longer course, more shots | Used since the 1980s; preferred over shorter agonist plans |
| Flare | Agonist starts at the beginning of the cycle, shots soon after | Varies | Used for some people expected to produce fewer eggs; mixed results |
| Natural cycle | No stimulation medicine | Few or none | One egg; cycles are stopped more often |
| Mini-IVF | Pills plus a small number of shots | Fewer shots | Fewer eggs; lower medicine cost per attempt |
The IVF antagonist protocol
This is a shorter plan. Your clinic checks you early in your cycle. Then you start stimulation shots. After about 6 days of shots, or once the follicles reach a size your clinic watches for, you add a GnRH antagonist shot. It works right away to stop early ovulation. Compared with the long protocol, it needs fewer days of treatment and less stimulation medicine.
The European Society of Human Reproduction and Embryology (ESHRE) updated its stimulation guideline in 2025. It still recommends the antagonist protocol over agonist protocols for the general IVF population, because it works about as well and is safer. Safer here mainly means a lower risk of ovarian hyperstimulation syndrome (OHSS), when the ovaries over-respond, swell and leak fluid. The warning signs are at the end of this page.
For people at risk of OHSS, the guideline recommends a GnRH agonist trigger combined with freezing all embryos, to lower the risk of severe OHSS. See freeze-all cycle. When all embryos will be frozen anyway, it says a progestin (a progesterone-like medicine) is probably an equal option for stopping early ovulation.
The long agonist protocol
In the long protocol, you start a GnRH agonist in the cycle before stimulation. In one common version, it starts around day 21 of that earlier cycle, so your body cannot release eggs early once the shots begin. Stimulation shots then start early in the next cycle, and you keep taking both until the trigger.
It takes longer, uses more stimulation shots and costs more. One review found it did better on egg growth and pregnancy rate. Even so, the 2025 ESHRE guideline prefers the antagonist protocol for most people, because of safety. When an agonist plan is used, it recommends the long version over short ones.
Why an agonist can both block and start ovulation
GnRH agonists can seem contradictory, and the answer is timing. When you first take one, it causes a burst of the hormones that drive egg growth and ovulation. If you keep taking it, it then quiets those same signals.
- In a long protocol, the agonist starts in the cycle before. By the time stimulation begins, the signals are quiet, so early ovulation is blocked.
- In a flare protocol, it starts with the cycle, so the first burst adds to the stimulation shots.
- As a trigger, a dose at the end of stimulation uses that first burst to finish maturing the eggs.
The flare protocol
In a flare protocol, the GnRH agonist starts at the beginning of the cycle, and the stimulation shots start a few days later. The early boost is the "flare," and the idea is to add it to the effect of the stimulation shots.
A low-dose version, the microdose flare, was developed for people expected to produce fewer eggs during stimulation. Studies of flare plans have mixed results.
Natural cycle IVF
Natural cycle IVF uses no stimulation medicine. The clinic collects the one egg your body grows that month. This is different from a natural frozen embryo transfer, which uses an embryo you already have.
The upside is fewer drugs. The downside is that your body may release the egg before retrieval, so natural cycles are stopped more often than stimulated ones.
A modified natural cycle adds a small amount of medicine, such as a GnRH antagonist, to lower the chance of early ovulation. For people expected to produce few eggs, the 2025 ESHRE guideline says it is probably not routinely recommended over standard stimulation. It adds that for people with very low ovarian reserve, doctors could choose one.
Mini IVF (minimal stimulation)
Mini-IVF uses lower amounts of medicine. In one well-known version, you take clomiphene pills together with a small number of stimulation shots. One study used about a quarter of the shots of a long agonist plan.
You may get fewer mature eggs. One review called it a cost-effective choice for older patients or people expected to produce fewer eggs. For people expected to respond poorly, the 2025 ESHRE guideline says clomiphene, alone or with shots, is probably an equal option to shots alone. For people with an expected normal response, it says a lower dose of stimulation shots is probably not recommended over a standard dose.
A lower medicine cost per attempt is not the same as a lower total cost. Fewer eggs per attempt can mean more attempts, each with its own visits, retrieval and lab fees. A Dutch trial of 404 patients compared two plans over a year. A mild plan moved one embryo at a time. A standard plan moved two. The mild plan took more IVF cycles (2.3 versus 1.7 on average). The mild plan still cost less in total, mainly because the standard plan led to more costly multiple pregnancies. That mild plan was not the clomiphene version of mini-IVF, so its costs can't be assumed for mini-IVF. Ask your clinic to compare the total expected cost of reaching your goal, not just one cycle's medicines.
Questions to ask about your protocol
Bring these to your next visit.
- Which protocol am I on, and why this one for me?
- What are its expected benefits, risks, cost and alternatives?
- Which medicines will I take, and on which days?
- When will the second medicine (agonist or antagonist) start?
- How will you decide when to trigger?
- What would make you change my plan partway through?
- Am I at higher risk of OHSS? What will you do about that?
- If this cycle does not go as hoped, would you try a different protocol next time?
When to get help
Call your clinic right away, day or night, if you have:
- A fever of 100.4°F (38°C) or higher.
- Bleeding your clinic did not tell you to expect, or a brown, watery discharge.
- Belly or pelvic pain that is sudden or severe, gets worse, or isn't eased by the pain relief your clinic recommended.
- Pain and bloating in your belly, or bloating that is getting worse.
- Nausea and vomiting, nausea that won't go away, or you can't keep fluids down.
- Peeing less than usual, even if you are not thirsty.
- Fast weight gain. Your clinic will tell you its number, but do not wait to reach a weight threshold if you feel unwell.
- Dizziness, lightheadedness, or feeling faint.
- Pain, redness or warmth in an arm or leg.
- Blood in your urine, or pain when you pee or poop.
Tell your clinic at your next contact about other milder symptoms that are new, don't go away or slowly get worse.
Key takeaway
Learn the name of your protocol, which medicine stops early ovulation, and the day each one starts. If a date or medicine on your calendar is unclear, ask your clinic before you take the next dose.