The contraceptive injection (Depo-Provera, Sayana Press): how it works and what to know

The contraceptive injection is a long-acting, progestogen-only method. The NHS describes the medicine as medroxyprogesterone acetate and says it is only available on prescription, through GP surgeries, contraception clinics and sexual health clinics.

Two brands are in use. Depo-Provera is the most common and is injected into a muscle in the buttock or upper arm. Sayana Press is injected under the skin of the abdomen or thigh, and can be self-administered after training, though availability varies by area.

The injection stops your body releasing an egg each month. It also thickens mucus at the neck of the womb so sperm cannot get through, and thins the lining of the womb, making it less likely that an egg would implant.

The NHS says it is more than 99% effective and that each injection protects you for at least 3 months. If given in the first 5 days after a period starts, protection is immediate; at any other point it takes 7 days, and another method such as condoms is needed in the meantime.

The first injection is normally given in the first 5 days of your period. After that the injections are repeated every 12 to 13 weeks. An injection a little early, before 12 weeks, is acceptable, though it might increase side effects. A gap of more than 14 weeks risks pregnancy.

If an injection is late beyond that point, you need other contraception straight away, and a late dose cannot be given if there is a chance you are already pregnant. Some people stop after 2 years and others carry on; the NHS recommends a bone density scan if you continue for longer than 2 years.

Most women and girls who have started their periods are eligible. The injection is not suitable if you have had an allergic reaction to it, are pregnant, trying for a baby or breastfeeding, have porphyria, have had hormone-sensitive cancer or have it in your family, have liver problems, have had a clot in a leg or lung, or have had a meningioma.

The prescriber will want to talk through high blood pressure, heart attack or angina, mental health conditions, migraines, diabetes, epilepsy, otosclerosis hearing loss, asthma, gallstones, lupus and any risk factors for osteoporosis before you start.

Periods can become irregular, heavier, shorter or lighter, or stop completely. Some people find heavy bleeding and period pain improve.

Weight gain is more common than weight loss. The NHS says it is typically less than 4lbs (2.2kg) over 1 to 2 years, though it can sometimes be up to 8lbs (4.4kg). Long-term use may weaken bones; regular exercise and enough calcium, from foods such as milk, cheese and leafy greens, help to strengthen them.

More than 1 in 100 people have effects such as headaches, stomach pain, bloating, nausea, mood changes, period changes, weight gain, reduced sex drive, dizziness, breast tenderness, back pain, tiredness, itchy skin, acne, hair loss, fluid retention and vaginal discharge changes. Rest, fluids, heat pads and simple remedies can help.

Serious problems affect fewer than 1 in 1,000 people. The NHS also notes a slight increase in the risk of meningioma after 3 or more years of use.

• Speak to a doctor or call 111 for fever with a burning feeling on passing urine, a painful swollen warm leg or arm, a red and painful swelling at the injection site, yellow eyes or skin with pale stools and dark urine, or blurred or double vision.

• Call 999 for sudden breathlessness with chest pain or coughing blood, sudden loss of vision, or signs of a severe allergic reaction.

• Report suspected side effects through the Yellow Card scheme.

Do not have the injection if you might be pregnant. If you discover a pregnancy after an injection, tell your doctor, though no harm is expected because the injection releases only a small amount of progesterone over a long period. It is safe in breastfeeding, with very little reaching the milk.

Other medicines will not usually affect it. The one named exception is St John's wort, which should not be taken with it. Alcohol is fine, and if you feel dizzy or tired avoid driving until it passes. Fertility should return after stopping, over a period that varies from a few months to over a year.