GnRH Antagonists

GnRH antagonists are a hormone-based fertility treatment. GnRH stands for gonadotropin releasing hormone. Gonadotropins are present in both men and women. As a medical fertility supplement, these drugs play a crucial role in fertility treatment for women because they control the body’s production of two other sex hormones, known as follicle stimulating hormone (FSH) and luteinizing hormone (LH), which control ovulation. GnRH antagonists have an effect similar to that of their close relatives, GnRH agonists; however, these fertility drugs function in slightly different ways.

How Antagonists Work

GnRH antagonists work directly on a part of the brain called the pituitary gland. These fertility supplements stop the pituitary gland telling the body to produce FSH and LH. This process is called “pituitary down-regulation.” This action impacts on ovulation because FSH is the hormone which encourages the follicles of the ovaries to develop and produce mature eggs, and LH is the hormone which tells the ovaries to release the eggs into the fallopian tubes, thus allowing a woman to get pregnant if her partner’s sperm reaches the egg. It may not seem logical that a fertility specialist would want to suppress the hormones which encourage ovulation and indeed, the aim of GnRH antagonists is not simply to stop ovulation, rather, these supplements are used to control the ovulation process.

Comparisons With Agonists

Whereas GnRH antagonists work directly on the pituitary gland – which then tells the body to stop producing FSH and LH - GnRH agonists work on another part of the brain called the hypothalamus. The hypothalamus is the part of the brain which first tells the pituitary gland - which then tells the body- to stop producing FSH and LH. Therefore, the main difference between antagonists and agonists is that antagonists intervene one stage later in the hormone-producing process. Given that both types of fertility supplement have the same outcome, namely, a cessation or decrease in FSH and LH production, the fact that the drugs work in different ways may not seem relevant to the patient. There are, however, practical treatment considerations arising from this difference. For example, GnRH antagonists take effect much more quickly (even within a couple of hours) than GnRH agonists. Indeed, agonists may take several days to work. Furthermore, when a patient’s treatment with GnRH antagonists stops, her FSH and LH production usually returns within 40 hours (depending on the dosage she was given), meaning that GnRH antagonists provide effective, short-term treatment.

Antagonists And Fertility

GnRH antagonists have a number of practical applications in fertility treatment, particularly in assisted reproductive technologies such as IVF and IUI.

IVF – During IVF, a woman often needs to take hormones which mimic the effect of FSH and LH, to stimulate the ovaries into producing and releasing eggs, which are then removed from the woman’s body, fertilized with sperm in a laboratory and then transferred back into the woman’s uterus where they may implant and grow into babies. The drugs given to stimulate the ovaries may encourage premature ovulation, which can cause the time-window for egg removal and fertilization to be missed. It can also leave the fertility specialist with only underdeveloped and poor quality eggs to work with, thus reducing the chances of a successful IVF cycle. This is why GnRH antagonists may be used during IVF treatment. By regulating the production of FSH and LH (it is a surge in LH which causes eggs to be released from the ovaries), GnRH antagonists can prevent the egg from being released too early.

IUI – GnRH antagonists may be administered as part of the intrauterine insemination process. Antagonists benefit the IUI patient in the same way as they do the IVF patient. If the patient has to take ovulation-inducing drugs, the GnRH antagonists enable controlled timing of ovulation, which helps the fertility specialist to recognize the best time to inseminate the woman with her partner’s or her donor’s sperm.

Endometrial Thinning – women who suffer from fertility problems due to endometriosis (whereby the reproductive organs can become distorted due to abnormal growth of endometrial tissue) may also benefit from treatment with GnRH antagonists supplements. By blocking FSH and LH production, antagonists also reduce estrogen levels in the body. Without estrogen, the abnormal endometrial tissue shrinks away.

Cancer Treatment – GnRH antagonists are being used as an experimental method of protecting a woman’s ovaries while she undergoes chemotherapy. The antagonists shut down the ovaries, which makes the ovaries less likely to become permanently sterile during the cancer treatment.

Types Of Antagonists

GnRH antagonists are sold under a number of brand names. Two of the best known types for women are cetrorelix and ganirelix. Cetrorelix is used to treat endometriosis and during IVF and IUI. This drug is administered as a weekly or daily injection. Ganirelix is used during IVF and is administered as a daily injection.

Side Effects

Women taking GnRH antagonists as a sole form of treatment risk experiencing menopausal symptoms. (These supplements effectively induce an artificial state of menopause.) Women who take antagonists at the same time as ovulation-inducing drugs generally do not experience menopause symptoms.


When used as part of IVF treatment, GnRH antagonists contribute to the risk of conceiving a multiple pregnancy (twins, triplets or more) which is risky for both the mother and her babies.

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