The contraceptive injection (often Depo-Provera) is a progestogen shot that protects against pregnancy for about 8–13 weeks at a time. It mainly stops ovulation, is highly effective, and needs nothing day to day — but it has two distinctive considerations: it's the method most linked with weight gain, and fertility can take longer to return after stopping.
How it works
According to the NHS and ACOG, the injection releases progestogen that:
- Stops ovulation (the main action),
- Thickens cervical mucus, and
- Thins the uterine lining.
Because it's progestogen-only, it avoids estrogen-related risks — an option for many who can't take the combined pill, including many who are breastfeeding.
There is more than one product. The NHS describes types given roughly every 13 weeks, including one designed to be injected under the skin that some people can be taught to give themselves at home, and a less commonly used type given about every 8 weeks. Which one you're offered depends on your country and clinic.
How often and how effective
You need a repeat injection roughly every 8–13 weeks depending on the product. With perfect, on-schedule use it's over 99% effective; typical-use effectiveness is a little lower because injections can be late. Getting it on time is the key to staying protected — see the full methods comparison.
Timing also governs when protection starts. The NHS explains that an injection given during the first five days of your menstrual cycle protects you immediately; given at any other point, you need additional contraception such as condoms for seven days.
One practical advantage worth knowing: the NHS notes the injection is not affected by other medicines in the way the pill and implant can be, and it keeps working normally through vomiting or diarrhoea.
What to expect at the appointment
The appointment itself is brief. A provider will confirm you're not pregnant, review your medical history and medicines, and ask about your periods and any plans for pregnancy. The injection is then given into a muscle — usually the upper arm or buttock — or, for the subcutaneous version, just under the skin of the thigh or abdomen. Expect brief soreness or a small bruise at the site.
Before you leave, do three things: note the date the next injection is due, ask how much leeway you have if you're late, and ask what to do if side effects trouble you. That last question matters more here than with other methods, because an injection cannot be undone once given.
Side effects by severity
Common and expected
- Irregular bleeding is common at first; many people's periods become lighter or stop with continued use, which is not harmful.
- Weight gain: the injection is the method most consistently associated with weight gain in research — see does birth control cause weight gain.
- Headaches, mood changes, acne, and breast tenderness.
Longer-term considerations
- Bone density: the NHS explains that long-term use can cause a small loss of bone mineral density, which generally recovers after stopping and is not linked to an increased fracture risk. Providers weigh this for people using it for several years, for younger users still building bone, and for anyone with other risk factors for osteoporosis.
- Side effects can't be switched off. Because the dose is already in your system, any unwanted effect may persist until it wears off — and sometimes for a period afterwards.
Reasons to contact a provider
Persistent heavy or prolonged bleeding, a hot, painful, or spreading reaction at the injection site, a new or severe headache, or low mood that isn't lifting all warrant an appointment rather than waiting for the next dose.
Who it suits — and who it doesn't
The injection can be a great fit if you want low-maintenance, estrogen-free contraception, don't want to think about it weekly or daily, and would find periods stopping a benefit rather than a worry. It also suits people who want their contraception to be entirely private, since there is nothing to store at home and nothing to find.
It may be less ideal if:
- You're planning a pregnancy within a year or two. The delayed return to fertility is the single most common reason people choose something else.
- Weight change is a priority concern for you. This is the one method where the association is consistent enough to raise up front.
- You have risk factors for osteoporosis, or would be using it for many years.
- You'd struggle to attend appointments on schedule. Effectiveness depends entirely on the interval being kept.
- You have certain medical conditions. The NHS lists unexplained bleeding between periods, some liver conditions, and a history of breast cancer among the reasons the injection may not be suitable.
How it compares to the alternatives
If the appeal is "nothing to remember", the implant and the IUD deliver the same convenience for longer, can be removed the moment they don't suit, and allow a fast return to fertility. That combination is why many providers raise them first. The injection's own advantages are that it involves no insertion procedure, is easy to start and stop between doses, and is unaffected by the medicines that can reduce the effectiveness of other hormonal methods. If daily action is acceptable but estrogen isn't, the progestogen-only pill covers similar ground with side effects you can stop at any time.
Cost and access
Availability differs by country. In the UK, the NHS provides the injection free of charge through GP surgeries and sexual health clinics, and in some areas people are taught to give the subcutaneous version themselves. In the United States, Planned Parenthood notes that most insurance plans cover the shot, with reduced-cost care available at community and publicly funded clinics. Wherever you are, factor in the recurring appointment: the method is only as accessible as your ability to return every few months.
What it doesn't do
It prevents pregnancy only — no STI protection. Use condoms to reduce STI risk.
The bottom line
The injection is convenient and highly effective, with the benefits of being estrogen-free, unaffected by other medicines, and needing attention only every few months. Its two stand-out trade-offs — possible weight gain and a slower return to fertility — plus the fact that a dose can't be reversed, are worth discussing with a provider before you start, especially if pregnancy is on your horizon. Compare it against the longer-acting options in our contraception guide. This article is general education and is not a substitute for individualized medical advice.
