Slow-release injections of progestogens for contraception have been used since the early 1960s – for almost as long as has oral contraception. Two different hormones are used at present.
1 Depot medroxyprogesterone acetate (DPMA, brand name Depo-Provera) This is injected into a muscle (usually the buttock) every 12 weeks. DPMA is available in Australia, and may be used for contraception by women for whom it is most suitable. It can also be used for the treatment of endometriosis, certain cancers and some uterine bleeding problems.
2 Norethisterone oenanthate (NET EN) is injected every eight weeks. It is not available in all countries.
How do injections work?
Mainly they work by stopping ovulation through inhibiting the release of FSH. Also, the progestogen makes the endometrium and cervical mucus anti-fertile; this effect is similar to but more steady than the effect of the mini-Pill.
How effective are contraceptive injections?
Slow-release injections of progestogens are the most effective reversible method of contraception. Theoretical efficacy is nearly total; the only mistake in using this form of contraception would be forgetting to have the next injection on time.
The main side-effect is unpredictable uterine bleeding. Most women will have no periods after six months of treatment.
During the first six months there may be frequent spotting or prolonged episodes of light bleeding. Other side-effects that have been reported are similar to those of the Pill, including moodiness and loss of libido, breast enlargement and tenderness, weight gain, aggravation of acne, bloatedness and headache. None of the oestrogenic side-effects of the Pill occur, such as facial pigmentation and nausea.
Two other problems may arise after stopping DPMA due to the residual effect of the injection which, though releasing too little progestogen to be relied on for contraception, can take months to be completely eliminated.
• When periods return, they may be heavy and irregular for a while.
• Return of fertility may be delayed for 12 months or more, though many studies have shown that after two years the pregnancy rate for women who have used DPMA is the same as for women of the same age who have used no contraception.
In case of failure, what is the effect on pregnancy?
There’s no evidence of an increase in the number of ectopic pregnancies or miscarriages. A few female babies conceived while the mother was using DPMA have slight enlargement of the clitoris. This returns to normal soon after birth. No other foetal abnormalities have been reported, but the number of pregnancies occurring is low because DPMA is such an effective contraceptive. There have been no effects on pregnancies of women who have recently been using contraceptive injections.
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