Yes, medroxyprogesterone prevents pregnancy, but only in its injectable form. The shot, commonly known by the brand name Depo-Provera, is one of the most effective reversible contraceptives available, with a pregnancy rate of just 0.3 per 100 women per year when given on schedule. The oral tablet form of medroxyprogesterone is prescribed for other conditions like abnormal uterine bleeding and is not used as birth control.
How the Injection Prevents Pregnancy
The Depo-Provera shot works primarily by stopping ovulation. The synthetic progesterone in the injection suppresses the hormonal signals that trigger the release of an egg each month. As long as blood levels of the drug stay above a certain threshold, ovulation simply doesn’t occur. Research shows that no participants ovulated when their blood concentration of the drug remained above a very low threshold, and ovulation only returned once levels dropped to near-negligible amounts.
The shot also thickens cervical mucus, making it harder for sperm to reach the uterus, and thins the uterine lining so that implantation of a fertilized egg becomes less likely. These backup mechanisms add extra protection even if ovulation were to occur unexpectedly.
Two Injectable Options
Two formulations are available in the United States. The intramuscular version delivers 150 mg and is injected into the upper arm or buttock. The subcutaneous version delivers 104 mg and is injected just under the skin, typically in the abdomen or thigh. The only real difference is the route of administration; both are given every 13 weeks and offer the same level of protection.
The subcutaneous version is FDA-labeled for administration by a healthcare professional, though some providers do prescribe it for self-injection at home on an off-label basis.
When Protection Starts
If you get your first injection during the first seven days of your menstrual period, protection begins right away. If you start at any other time in your cycle, you’ll need to use a backup method like condoms for seven days while the drug reaches effective levels.
After that initial window, you’re protected continuously as long as you keep getting your shots on time. Each injection is due every 13 weeks, with a grace period of up to 2 weeks. That means you can be up to 15 weeks from your last injection and still receive your next shot without needing backup contraception. Beyond that window, you may have already started to ovulate, since the drug’s suppressive effect can wear off as early as 14 weeks after an injection.
Bone Density: The Main Safety Concern
The most significant long-term concern with the shot is bone density loss. The drug lowers estrogen levels, which leads to a gradual decrease in bone mineral density similar to what happens during menopause. In clinical studies, women who used the injection for up to five years lost an average of 5 to 6% of bone density at the spine and hip, compared to no significant change in women who weren’t on the drug.
The losses accumulate with time. After one year, average spine density dropped about 2.9%. By five years, it reached about 5.4%. Adolescents and young adults are especially vulnerable because their bones are still building toward peak density. In studies of adolescent users, bone density at the hip and thigh bone had not fully recovered even five years after stopping the injection.
For adults, recovery was partial. Bone density at the hip, thigh bone, and spine moved back toward baseline within about two years of stopping, but didn’t fully return in women who had used the shot for more than two years. The FDA labeling recommends limiting use to two years unless other birth control methods aren’t suitable, and suggests bone density monitoring for women who need to stay on it longer.
Other Common Side Effects
Irregular bleeding is the most frequent side effect, especially in the first few months. Some women experience spotting or prolonged periods, while others stop having periods altogether after several months of use. By one year, many users have very light or no periods at all, which some consider a benefit rather than a side effect.
Weight gain is frequently reported, though the amount varies widely from person to person. Other possible effects include headaches, mood changes, decreased sex drive, and bloating.
How Long Fertility Takes to Return
Unlike most other contraceptives, the shot causes a significant delay in the return of fertility after you stop using it. On average, ovulation returns about seven months after the last injection (roughly 219 days), though the range is wide. Some women ovulate as early as 3.5 months after their last shot, while others may wait nearly 12 months. The median time is about 212 days.
This delay is not permanent, and the drug does not cause infertility. But if you’re planning to get pregnant within the next year, it’s worth knowing that the timeline is less predictable than with pills, patches, or IUDs. There’s currently no evidence that longer use of the shot extends the delay further, but that specific question hasn’t been well studied.
Who Should Not Use the Shot
The injection is contraindicated for women with a known or suspected pregnancy, a history of breast cancer, active or past blood clots (deep vein thrombosis or pulmonary embolism), a history of stroke or heart attack, liver disease, undiagnosed abnormal genital bleeding, or a previous severe allergic reaction to the drug. If any of these apply to you, other contraceptive options would be safer.
Medications That May Interact
Several medications can potentially reduce the effectiveness of medroxyprogesterone or create problematic interactions. Certain seizure medications (carbamazepine, phenytoin, and others), some HIV treatments (efavirenz, darunavir, etravirine), and the antifungal griseofulvin are among the drugs flagged for possible interaction. Smoking tobacco is also noted as a concern. If you take any prescription medications regularly, it’s worth confirming there’s no conflict before starting the shot.
Oral Medroxyprogesterone Is Not Birth Control
It’s important to distinguish between the injectable and oral forms. Oral medroxyprogesterone (brand name Provera) is prescribed in tablet form for conditions like irregular periods, endometriosis, and abnormal uterine bleeding. It is not approved or effective as a contraceptive. The injectable version delivers the drug at much higher, sustained levels that are necessary to reliably suppress ovulation over three months. Taking the oral tablets will not prevent pregnancy.