Depo-Provera is a hormonal birth control shot that prevents pregnancy by stopping ovulation. It delivers a synthetic form of progesterone into your body, where it slowly releases over three months, keeping hormone levels high enough to shut down your monthly egg release. With perfect use, it’s 99.8% effective. In typical real-world use, where some people miss or delay their shots, about 6 out of 100 users become pregnant in the first year.
The Three Ways It Prevents Pregnancy
The active ingredient in Depo-Provera is a lab-made version of progesterone, the hormone your body normally produces after ovulation. When injected, it works through three overlapping mechanisms that each reduce your chances of pregnancy independently.
First, it suppresses the hormonal signals from your brain that tell your ovaries to mature and release an egg each month. Without that signal, ovulation simply doesn’t happen, and no egg means no pregnancy. Second, it thins the lining of your uterus, making it less hospitable for a fertilized egg to implant. Third, it thickens the mucus at your cervix, creating a physical barrier that makes it harder for sperm to reach an egg even if one were released.
What Happens After the Injection
Within 24 hours of receiving the shot, blood levels of the hormone rise high enough to begin suppressing ovulation. The drug is deposited as a slow-release reservoir in your muscle or fat tissue, gradually entering your bloodstream over the following weeks. By the end of the 12-to-14-week dosing window, levels have dropped but typically remain just above the threshold needed for contraception. The hormone has a half-life of roughly 40 days, meaning it takes that long for your body to clear just half of what remains.
This slow clearance is what makes the shot so effective at preventing pregnancy, but it also means the hormone lingers in your system well after you stop getting injections.
Two Versions: IM and SubQ
Depo-Provera comes in two formulations. The original version delivers 150 mg by intramuscular injection, typically into the upper arm or buttock. A newer, lower-dose version delivers 104 mg just under the skin (subcutaneously) in the thigh or abdomen. Both are given once every three months, and both are equally effective at preventing pregnancy. The subcutaneous version uses a smaller needle and a lower dose because the drug absorbs differently through fat tissue than through muscle.
Timing and the Grace Period
The standard schedule is one injection every 13 weeks. If life gets in the way, the CDC says you can receive your repeat shot up to two weeks late (15 weeks from the last injection) without needing backup contraception. Beyond that window, you’ll need a pregnancy test before the next shot and should use condoms or another backup method in the meantime.
If you’re starting Depo-Provera for the first time and receive the injection within the first seven days of your period, it’s effective immediately. If you start at any other point in your cycle, you’ll need backup contraception for the first seven days.
Weight Changes
Weight gain is one of the most common concerns with Depo-Provera, and the data shows it’s a real but modest effect for most users. In clinical trials, the average weight gain after one year was about 1.4 to 1.7 kg (roughly 3 to 4 pounds). Over three years, that number climbed to a median of about 4.4 to 5.1 kg (roughly 10 to 11 pounds). These are averages, so some people gain more, some gain less, and some don’t gain at all. The mechanism isn’t fully understood, but increased appetite appears to play a role.
Bone Density Loss
Depo-Provera carries an FDA black box warning about bone mineral density, the strongest type of safety warning available. The shot suppresses your body’s natural estrogen production (a side effect of shutting down the ovulation cycle), and estrogen is essential for maintaining strong bones. The longer you use Depo-Provera, the more bone density you lose, and that loss may not fully reverse after stopping.
Because of this, the FDA recommends Depo-Provera as a long-term method (longer than two years) only when other birth control options aren’t adequate. This is especially relevant for teenagers and young adults who are still building peak bone mass, since it’s unknown whether early use could increase the risk of osteoporosis later in life. If you have other risk factors for bone loss, such as a family history of osteoporosis, smoking, or low calcium intake, this is worth discussing before choosing the shot.
Other Side Effects
The most noticeable change for most users is to their menstrual cycle. Irregular bleeding and spotting are common in the first few months. By the end of the first year, roughly half of users stop having periods entirely. This isn’t harmful; it’s simply a result of the uterine lining becoming so thin that there’s nothing to shed. For many people, the absence of periods is actually a reason they prefer Depo-Provera over other methods.
Other reported side effects include headaches, mood changes, decreased sex drive, and breast tenderness. Because the shot can’t be “undone” once injected, any side effects you experience will persist until the hormone clears your system, which takes several months.
How Long Fertility Takes to Return
Unlike the pill or an IUD, fertility does not bounce back immediately after stopping Depo-Provera. The slow-clearing hormone keeps suppressing ovulation for weeks or months after your last shot. About half of people who stop the shot conceive within 10 months. Some studies have found the average time to pregnancy is roughly five to seven months after the drug wears off, though individual timelines vary widely. If you’re planning a pregnancy in the near future, this delay is worth factoring into your decision.
Benefits Beyond Contraception
Because Depo-Provera thins the uterine lining and often stops periods altogether, it’s frequently prescribed for people with heavy or painful menstrual bleeding, endometriosis, or conditions where reducing menstrual blood loss is medically beneficial. It’s also used in people who can’t take estrogen-containing birth control due to migraine with aura, blood clot risk, or other health conditions, since Depo-Provera contains only a progestin. Researchers are also actively studying whether the shot can reduce pain crises in women with sickle cell disease, where menstrual hormonal shifts may trigger episodes.