Injectable Contraceptives

The two injectable contraceptives that to date have been tested fairly rigorously are depot medroxyprogesterone acetate (DmpA) and norethindrone enanthate (NET-EN). Both of these are progestogens, or progestins, and between them they are currently being used in eighty countries around the world.


This drug is most commonly marketed under the brand name Depo-Provera. A similar drug was first synthesized in 1958, and used (unsuccessfully) to treat threatened miscarriage. It was soon discovered that large doses in injection form had a contraceptive effect. Like most other progestogen-only contraceptives, DMPA has several effects which combine to prevent pregnancy. It works on the pituitary to inhibit the secretion of LH and FSH, which interrupts the chain reaction that leads to ovulation. It also acts on the tissues of the fallopian tubes, and on the cervical mucus to help prevent sperm penetration. In addition it atrophies the endometrium, which prevents implantation of any ovum that might be fertilized.

The drug is administered by injection, generally in a regime of 150mg every three months. The efficiency of this regime varies between 0-1.2 pregnancies per 100 woman/ years. A less usual regime of 400-450mg every six months has a slightly higher pregnancy rate โ€” up to 3.8 per 100 woman/years in one study. The drug has to be given within 5-7 days of the beginning of a period, otherwise the woman may already be pregnant. The drug actually gives more than three months’ protection per 150mg, but is overlapped to prevent a sudden drop in blood levels and to give a little leeway in case the injection is delayed. One disadvantage of the method is that it cannot be reversed until that particular dose of the drug has worn off.

The most common side-effect is disturbance of the menstrual cycle. This can take the form of excessive bleeding or of absence of periods, which often disappear completely after 9-14 months on the drug. Of course for some women this is a boon rather than a disadvantage, but others may begin to fear that they are pregnant. Bleeding can be induced by giving oestrogen supplements, but this cancels out one of DMPA’S main benefits โ€” that it gives the security of a hormonal method without the health risks associated with oestrogen-containing pills. Other side effects can include headache, abdominal bloating, mood changes, decreased libido, dizziness, weight gain, depression, and allergic reactions. DMPA has very little effect on blood pressure.

DMPA does not inhibit lactation once breastfeeding is well established, and there is so far no evidence that it can harm a breastfed infant, although it does cross into the milk to some extent. It has been recommended by some bodies that it should not be given until at least six weeks after giving birth. Practitioners tend to use the same absolute contraindications for DMPA as for the combined pill, with the addition of undiagnosed uterine bleeding. So far, however, it has not been shown to increase the risk of any lethal conditions.

There has been a great deal of controversy over trials of the drug done on beagle bitches and monkeys, which seemed to imply that the drug might act as a catalyst for breast tumours or endometrial cancer. The beagle trial is now suspect as many medical bodies (including the UK Committee for the Safety of Medicines and the WHO) have concluded that beagles are not suitable models for predicting human reactions to the drug. Nevertheless, as a precaution, all DMPA users should practise conscientious breast self-examination.

The monkey trial also seems to have been a false alarm, partly because the two monkeys affected were on fifty times the human dose, and partly because the tumours appear to have arisen from a cell type not found in humans. Studies on other animals suggest that DMPA may harm a foetus conceived while using the drug. These results are still inconclusive.

One of the less desirable effects of DMPA is that it delays the return of fertility. The length of the delay is unpredictable, but it is generally agreed to average about 7 months (some studies say 5.5, some say 10). By 12 months after stopping the drug, over 20% of women trying to conceive had still not done so โ€” by 24 months the percentage was 7.9%. Infertility is still not considered a serious problem by many doctors working on the trials, although presumably some of the women concerned have a different opinion.

DMPA has been the centre of some controversy in this country, but has recently been given Department of Health approval for use by women who are unsuited to other methods. It has been used with considerable success in certain parts of the country. The WHO and IPPF (International Planned Parenthood Federation) are happy with DMPA on the present evidence, but the USA has not approved it yet; it feels that so far there is not sufficient evidence to say that the drug is harmless. Also some authorities are not happy about the potential hazards of the irregular bleeding problems it induces. If these could be conquered it would be more acceptable to both doctors and patients.


NET-EN is not as well-known as DMPA, and has not been so widely used and tested. Nevertheless many women have taken part in trials, and the drug so far has some attractive advantages over DMPA.

The standard dose is not quite so long-acting as DMPA. It has been suggested that it is administered every twelve instead of every thirteen weeks, but even this gives a rather too high pregnancy rate at the end of the injection interval. Some clinicians are working on a regime of 8-week intervals for 6 months, followed by 12-week intervals from then on; others prefer a standard 10-week interval. The four major trials have given pregnancy rates of 1.5-5.2 for 13week intervals; most of these pregnancies occur in the third month of the first injection.

The drug seems to inhibit ovulation completely at the beginning of the interval, although there is some evidence that it can return during the third month. The cervical mucus is less hospitable to sperm, and some, but not all, studies show that the endometrium is affected so that it discourages implantation of a fertilized ovum.

Side-effects include possible menstrual disruption, mainly spotting, decreased flow or irregular periods. The two extremes of absence of periods and heavy bleeding are not so common with NET-EN as with DMPA, which may make NET-EN more acceptable to many women. Other side effects can include headache (fairly frequently), also anxiety, abdominal discomfort and loss of libido. There does seem to be a measurable weight gain for most women using NET-EN; also the drug seems to be stored better by overweight women than by underweight ones, who apparently run a higher risk of becoming pregnant! The drug also appears to help protect against the infection, vaginal moniliasis.

Fertility is re-established more quickly after NET-EN than after DMPA, with an average delay of 1-4 months. So far no abnormalities have been found in foetuses conceived accidentally while using NET-EN, but further research is needed before this can be stated definitely. The drug does not appear to have any effect on milk production during lactation, but some of the drug does cross into the milk.

Advantages of injectables

Injectables are very easy and reliable methods of contraception, although they do have some failures. Once the drug is administered, nothing more needs to be done until it is time for the next injection. As a result, there is little or no difference between the theoretical failure rate and the actual failure rate. As with other progestogen-only contraceptives, injectables avoid most of the serious complications associated with oestrogen-containing pills. Effectiveness often continues even when the next injection is a little late, and is good in all respects for forgetful women. The drugs do not appear to suppress lactation, and using a regime of injectables may decrease the incidence of anaemia through their interruption of the menstrual cycle.


Injectable contraceptives cannot be reversed until the effect of the latest injection has worn offโ€” this means that they are not under the full control of the user. Menstrual disruption is a common side-effect of the drugs, and the return of fertility can be delayed considerably after the drug is stopped. In common with other progestogen-only contraceptives, they may work partly by preventing implan-

tation of a fertilized ovum, which means that the Christian should consider very carefully before choosing this method of contraception using the above drugs. This method of contraception is only possible with medical supervision.


Injectable contraceptives as a theory have a promising future; for women who are happy with hormonal methods, these could provide very effective long-term protection with the minimum of participation and bother. In practice, neither of the two main injectables is trouble-free, and at the moment neither is generally available in this country.