Norgestrel

What norgestrel does

Norgestrel is a progestin, a synthetic form of the hormone progesterone, and in a contraceptive pill its whole purpose is to stop pregnancy before it begins. It works on three fronts: it holds back the monthly release of an egg from the ovaries, thickens the mucus at the neck of the womb so sperm have a harder time getting through, and keeps the womb lining thin. Taken as a daily tablet, that is what makes the method reliable. Pregnancy is all it prevents — condoms are still what guard against sexually transmitted infections.

Ovral, the norgestrel tablet sold here

Norgestrel is sold here as Ovral, a combined oral contraceptive tablet that pairs norgestrel with ethinyl estradiol, an oestrogen. Two strengths are shown on its product page: 0.15 mg norgestrel with 0.03 mg ethinyl estradiol, and 0.5 mg norgestrel with 0.05 mg ethinyl estradiol. Whichever strength is chosen, it is one tablet daily at a consistent hour, taken in the order printed on the pack. That timing keeps the hormone level steady; a tablet at breakfast one morning and at bedtime the next stretches the gap further than a daily pill is meant to have.

What happens if a tablet is late, or comes back up?

Combined pills forgive a few hours, not a whole day. A tablet taken less than 24 hours after the usual time keeps protection intact. Once a whole day has gone by, or more than one tablet has been missed, take the most recent missed one, carry on with the rest of the pack, and use condoms for the next seven days. Vomiting or diarrhoea within about four hours of a tablet means it may not have been absorbed, so condoms cover the following two days.

What to expect, and what needs urgent attention

During the first few months the usual complaints are nausea, tender breasts, headaches, low mood and spotting, and most fade as the body adjusts. Spotting early on does not mean the pill has stopped working. Clots are rare but serious: a painful swollen calf, crushing chest pain, sudden breathlessness, sudden loss of vision or a severe headache unlike any before need urgent treatment. Anyone with a history of blood clots, breast cancer or liver disease does not use a pill that contains an oestrogen.

Medicines and supplements that can make it fail

Some medicines speed up the liver's handling of the pill's hormones and leave too little in the blood. The main ones are rifampicin and rifabutin, the antifungal griseofulvin, the epilepsy medicines carbamazepine, phenytoin and topiramate at higher doses, and St John's wort. That effect lasts through the course and for 28 days after the last dose. Other antibiotics — penicillins, cephalosporins, macrolides, tetracyclines, quinolones, metronidazole — do not reduce the protection a contraceptive pill gives.