GnRH agonists are modified versions of gonadotrophin-releasing hormone - a naturally occurring hormone that helps to control the menstrual cycle.

The GnRH agonists reduce or eradicate endometrial implants by suppressing ovulation and the production of oestrogen by the ovaries. The low levels of oestrogen in the body mean that the endometrial implants are no longer stimulated to grow and breakdown each month so they gradually shrink or 'dry up'.

Possible side effects

The most commonly reported side effects are those associated with menopause, and include:

• Hot flushes / night sweats
• Vaginal dryness
• Mood changes / depression
• Acne
• Muscle pains
• Decreased breast size

The GnRH agonists also cause a marked decrease in bone density (i.e. the thinning of the bones). Much of this loss of bone density is reversed within six months of completing treatment and it is usually completely or almost completely reversed within twelve to eighteen months of completing treatment. Nevertheless, this loss of bone density can be serious as it can predispose you to osteoporosis later in life or even during treatment if you are already at risk of developing osteoporosis. Make sure you discuss with your health practitioner whether or not you should have a bone density scan (DEXA) before beginning treatment. Sometimes hormones are added back to relieve severe symptoms without reducing the effect of the GnRH agonist or endometriosis.

Content Updated August 31, 2009