How it works
It is a depot progestogen, injected into a muscle or under the skin, which releases slowly over about thirteen weeks. It blocks ovulation and thickens cervical mucus. It contains no oestrogen.
It is the most discreet method there is: nothing to carry, nothing to store, nothing to hide between injections. That argument genuinely matters in some personal situations, and explains part of its use.
The schedule
One injection every twelve to thirteen weeks, given by a professional or, for some subcutaneous presentations, self-administered. Keeping to the interval is what the effectiveness rests on: a delay of a few weeks opens a window of risk.
Effectiveness
| Pregnancies per 100 women per year | |
|---|---|
| Perfect use | around 0.2 |
| Typical use | around 4 |
The gap comes from late or unrenewed injections.
What it changes
- Periods: they are irregular for the first few months, then often stop as the injections continue (WHO/JHU handbook). This amenorrhoea is harmless, and some users seek it.
- Bone: the injection comes with a reversible fall in bone mineral density. That calls for a specific discussion in adolescents, whose bone mass is still being built, and with very prolonged use.
- Weight: gradual weight gain is common, around 1 to 2 kg a year on average (WHO/JHU handbook).
The constraint unique to this method
Once the injection is given, it cannot be taken back. If the effects are poorly tolerated, you wait out the thirteen weeks. That is the essential difference from the pill, the implant or the ring, which can be stopped overnight — and the reason a first injection deserves a real discussion, not just a prescription.
Return of fertility is delayed
Allow on average nine to ten months after the last injection for ovulation to return, sometimes longer. That delay is not infertility and predicts nothing, but it makes the injection a poor fit if pregnancy is planned within the year.