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.
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.
Side effects to weigh
- Irregular bleeding is common at first; many people's periods become lighter or stop with continued use.
- Weight gain: the injection is the method most consistently associated with weight gain in research — see does birth control cause weight gain.
- Bone density: long-term use can cause a temporary reduction in bone mineral density, which generally recovers after stopping. Providers consider this for long-term or younger users.
- Other effects can include headaches, mood changes, and acne.
Who it suits
The injection can be a great fit if you want low-maintenance, estrogen-free contraception and don't want to think about it weekly or daily. It may be less ideal if you're sensitive to weight changes, planning pregnancy soon, or have specific bone-health risks. A provider can help you weigh these.
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 and needing attention only every few months. Its two stand-out trade-offs — possible weight gain and a slower return to fertility — are worth discussing with a provider before you start, especially if pregnancy is on your horizon.
