Progestogenonly Contraception

The oral formulation ('mini-pill') is taken every day; it must be taken at the same time each day (to within 3 hours). Oral progestogen-only contraception is less effective but safer (no effect on blood coagulation) than combined formulations.

Subdermal implantations that release hormone for several years are in use; they can be removed surgically if adverse effects develop or pregnancy is desired. For example, a flexible rod containing etonorgestrel (Implantan) inserted into the lower surface of the upper arm provides contraception for 3 years (2 years for overweight women because they have lower blood concentrations). The rod must be removed when its effective period has elapsed.

Intramuscular progestogen. A 3-month depot injection is equal in efficacy to the combined pill and is an alternative. It works by inhibiting ovulation, and also renders cervical mucus impenetrable to sperm.

Progestogen-only contraception is particularly appropriate to women having an absolute contraindication for oestrogen, e.g. history of thromboembolism, smokers over 35 years (who refuse to give it up), and for diabetics. Hypertension is not an absolute contraindication to the more effective combined pill since only a proportion of women have oestrogen dependent hypertension (and often such women are normotensive until exposed to increased levels of oestrogen). It is used by lactating women as it interferes with the milk less than the combined pill.

A missed oral dose allows even less latitude than the combined pill. If a dose is more than 3 hours late it should be taken at once and a barrier method used for 7 days. Act similarly where there has been vomiting, severe diarrhoea, or if an enzyme-inducing drug has been taken.

A significant limitation to the use of the progestogen-only pill is erratic uterine bleeding which many women understandably dislike. There may be no bleeding for months or there may be frequent and irregular bleeding. Ectopic pregnancy may be more frequent due to a fertilised ovum being held up in a functionally depressed fallopian tube. Other adverse effects are generally less than the combined pill (blood coagulation is unaffected), data on breast cancer are conflicting but are largely reassuring. Ovarian cysts occur more frequently in progesterone-only pill users.

The progestogens used (alone) orally include norgestrel, levonorgestrel, ethynodiol, norethisterone, desogestrel (e.g. Noriday, Micronor, Femulen). Medroxyprogesterone (Depo-Provera) (t% 28 h) is a sustained-release (aqueous suspension) deep i.m. injection given 3-monthly. When injected between day 1 and day 5 of the menstrual cycle, contraception starts immediately. If given after day 5, a barrier contraceptive is needed for 7 days. Depo-Provera can be started within 5 days of childbirth or abortion; however, starting it so soon after childbirth may cause heavy bleeding and so waiting until 6 weeks postpartum is probably better.

Medroxyprogesterone acetate and its metabolites are excreted in breast milk, so women who breastfeed should wait until 6 weeks post partum before starting Depo-Provera, when the infant's enzyme system should be more mature. Norethisterone enantate 200 mg (Noristerat) is shorter acting than Depo-Provera, 8 weeks, and is used to provide contraception after administration of the rubella vaccine, and until a partner's vasectomy has taken effect. It can also be used in the longer-term but only on a 'named patient' basis.

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