The contraceptive injection is one of the most effective reversible birth control methods available. With perfect use, it has a failure rate of less than 1%. With typical use, which accounts for late or missed injections, about 4 in 100 people will become pregnant over a year. That puts it well ahead of the pill, patch, and ring in real-world effectiveness, primarily because it removes the daily or weekly compliance factor.
How the Injection Prevents Pregnancy
The injection delivers a synthetic form of progesterone that works on multiple levels. Its primary effect is suppressing ovulation: your ovaries simply don’t release an egg. As a secondary safeguard, the hormone thickens cervical mucus, making it much harder for sperm to reach an egg even in the unlikely event one is released. These overlapping mechanisms are what make the injection so reliable when given on schedule.
Perfect Use vs. Typical Use
The gap between less than 1% and 4% failure rates comes down to one thing: timing. Each injection is scheduled every 13 weeks (about 3 months). If you show up on time, the protection is nearly absolute. But in everyday life, people miss appointments, forget to reschedule, or can’t get to a clinic on the right day. Each late injection is a window where protection drops.
The CDC allows a grace period of up to 2 weeks past your scheduled date. That means you can receive your next injection as late as 15 weeks after the previous one without needing backup contraception. If you’re more than 2 weeks late, you can still get the injection, but you’ll need to use condoms or abstain for the following 7 days while the hormone levels build back up.
Body Weight Does Not Reduce Effectiveness
Unlike some other hormonal methods, the injection works equally well regardless of your weight. A pharmacokinetics study found that while hormone levels were somewhat lower in women with a BMI of 30 or above, they still remained well above the threshold needed to suppress ovulation. A larger study of 846 women found that baseline body weight had no relationship to contraceptive failure. The Faculty of Sexual and Reproductive Healthcare confirms that the effectiveness of the injection is not affected by body weight or BMI.
Two Types of Injection
The traditional injection is given into a muscle, usually in the upper arm or buttock, at a dose of 150 mg. A newer formulation, designed for injection just under the skin (subcutaneous), achieves the same level of effectiveness at a 31% lower dose of 104 mg. The lower dose works because the hormone absorbs more slowly from under the skin, maintaining effective blood levels over the same 3-month window.
The subcutaneous version is available in a prefilled, single-use device approved for self-injection at home. This is a practical advantage for people who find it difficult to visit a clinic every 13 weeks, and it may help close the gap between perfect and typical use rates by making on-time dosing easier.
Changes to Your Period
The most common side effect is a change in bleeding patterns. In the first few months, irregular spotting or breakthrough bleeding is typical. Over time, periods tend to become lighter and less frequent. After one year of use, about 52% of users stop having periods altogether. For many people this is a welcome benefit, though it can be unsettling if you’re not expecting it. The absence of a period on the injection is not harmful; it simply reflects the hormone’s suppression of the uterine lining.
Weight Gain: What the Data Shows
Weight gain is one of the most commonly cited concerns about the injection, but the research paints a more modest picture than many expect. A comparative study tracking users of the injection, hormonal implants, and the pill over 12 months found that injection users gained an average of just 0.1 pounds. Pill users actually lost about 2 pounds on average, and implant users lost about 1.8 pounds. The difference was statistically significant but small in absolute terms. Some individuals do gain more weight, but the average effect over the first year is minimal.
Medications That Can Interfere
Certain medications speed up how your liver processes hormones, which can lower the injection’s effectiveness. The FDA lists several that warrant caution:
- Some epilepsy medications, including carbamazepine, phenytoin, and topiramate
- Rifampin, an antibiotic used for tuberculosis
- St. John’s wort, an herbal supplement for mood
- Certain HIV medications, including protease inhibitors
If you take any of these, your provider may recommend additional contraceptive protection or an alternative method that isn’t affected by these interactions.
How Long Fertility Takes to Return
The injection is fully reversible, but fertility doesn’t bounce back immediately. Because each dose is designed to last about 15 weeks, there’s a built-in delay before the hormone clears your system. After that, ovulation still needs to resume and normalize. Research from a large study in Thailand found that the median time from the last injection to conception was about 9 months. That figure includes the 15 weeks of residual effect from the final dose, plus roughly 5.5 additional months of waiting.
This is notably longer than the return to fertility after stopping the pill or removing an IUD, where most people ovulate within one to three months. It doesn’t mean the injection causes lasting infertility. It means that if you’re planning to conceive in the near future, you may want to switch to a shorter-acting method a few months ahead of time to give your cycle a chance to restart.
How It Compares to Other Methods
At a 4% typical-use failure rate, the injection sits between the most effective reversible options (IUDs and implants, which fail less than 1% of the time even with typical use) and user-dependent methods like the pill, patch, and ring (which fail about 7% of the time with typical use). Its main advantage over daily or weekly methods is simplicity: you only need to think about contraception four times a year. Its main disadvantage compared to IUDs and implants is that it still requires you to keep an appointment schedule, and once injected, it can’t be “removed” if you experience side effects you don’t like. You simply have to wait for it to wear off.