What is the Depo (Depot medroxyprogesterone acetate) shot and how does it work as a contraceptive method?

Depo Shot (Depot Medroxyprogesterone Acetate)

The Depo shot is a highly effective progestin-only injectable contraceptive containing 150 mg medroxyprogesterone acetate given intramuscularly (or 104 mg subcutaneously) every 13 weeks, preventing pregnancy primarily by inhibiting ovulation, with a typical use failure rate of only 0.3%. 1, 2

Mechanism of Action

The Depo shot works through multiple contraceptive mechanisms 3:

• Inhibits gonadotropin secretion, which prevents follicular maturation and blocks the midcycle LH surge necessary for ovulation 2, 3
• Suppresses ovulation by maintaining serum progesterone levels below 0.4 ng/mL for several months after injection 3
• Thins the endometrium, creating an atrophic endometrial lining with small, straight glands and decidualized stroma 3
• Thickens cervical mucus, making it viscid and impenetrable to sperm 3

Pharmacokinetics and Duration

After a single 150 mg intramuscular injection 2, 3:

• MPA is detectable in serum within 30 minutes
• Concentrations plateau at approximately 1.0 ng/mL for about 3 months
• Peak levels reach 1-7 ng/mL within 3 weeks
• The apparent half-life is approximately 50 days
• MPA may remain detectable for up to 9 months after a single injection in some women
• Ovulation resumes when MPA levels fall below 0.1 ng/mL

Administration Schedule

Injections must be given every 13 weeks (3 months), with a grace period allowing administration up to 2 weeks late (15 weeks total) without requiring backup contraception. 4, 5, 6

• Intramuscular formulation: 150 mg deep IM injection 1, 2
• Subcutaneous formulation: 104 mg subcutaneous injection (can be self-administered) 1, 5
• Backup contraception required: Only for the first 7 days after the initial injection if started more than 7 days after menses began 4

Contraceptive Efficacy

Depo-Provera ranks among the most effective reversible contraceptives 1, 2:

• Perfect use failure rate: 0.2% per year
• Typical use failure rate: 0.3% per year
• Efficacy is independent of daily adherence, coital timing, or body weight (though lower body weight women conceive sooner after discontinuation) 2, 7

Clinical Advantages

Non-Contraceptive Benefits 1

• Reduces dysmenorrhea and provides protection against iron-deficiency anemia through decreased menstrual bleeding
• Decreases endometrial cancer risk with prolonged use
• May raise seizure threshold in epileptic patients
• May decrease sickle cell crises

Practical Benefits

• Contains no estrogen, making it suitable for women with contraindications to estrogen including history of thromboembolism, migraine with aura, cardiovascular disease, or active/moderate SLE 4, 1
• Convenient: Requires only quarterly visits rather than daily pill-taking 1
• Independent of user memory between injections, eliminating the "fear of forgetting" associated with daily methods 8
• No effect on blood pressure or thrombosis risk, unlike combined hormonal contraceptives 8

Special Populations

Women with Rheumatic and Musculoskeletal Diseases

DMPA is safe in most women with RMD, with specific exceptions for those with positive antiphospholipid antibodies or high osteoporosis risk. 4

• Strongly recommended over combined estrogen-progestin contraceptives in SLE patients with moderate or severe disease activity 4
• Contraindicated in women with positive aPL due to baseline thrombosis risk 4
• Use caution in women at high risk for osteoporosis 4

HIV-Infected Patients

DMPA is generally safe for HIV-infected patients 4:

• HIV infection is not a contraindication (CDC category 2) 4
• DMPA levels are not reduced by antiretroviral agents including efavirenz, zidovudine, lamivudine, nevirapine, and nelfinavir 4
• Women should be informed that progestin-only injectables may or may not increase HIV transmission risk to partners, and condom use should be encouraged 4

Major Side Effects and Management

Menstrual Irregularities

Nearly all patients initially experience menstrual cycle changes, including irregular bleeding, spotting, or heavy bleeding, but these typically improve over time with most users eventually becoming amenorrheic. 1, 8, 9

• Pre-injection counseling about expected menstrual changes significantly improves continuation rates 1
• For spotting or light bleeding, NSAIDs for 5-7 days may be considered 5
• Short courses of estrogen or shorter injection intervals can improve troublesome bleeding 8

Weight Gain

Weight gain is common but variable between individuals 1:

• 21% of adolescents experience early weight gain (>5% at 6 months), and these "early gainers" continue gaining weight over 18 months 1, 6
• Baseline weight and BMI measurement is useful for monitoring over time 6

Bone Mineral Density Loss

The American College of Obstetricians and Gynecologists does NOT recommend limiting DMPA use to 2 years, as the benefits of preventing unwanted pregnancy generally outweigh the risks of bone density loss, and BMD substantially recovers after discontinuation. 1, 6

• No routine bone density monitoring is required, even after prolonged use beyond 2 years 6
• BMD assessment may be considered only if additional osteoporosis risk factors are present (heavy alcohol use, smoking) 6
• All patients must receive counseling on skeletal health measures 1, 6:
  • Daily calcium intake of 1,300 mg
  • Daily vitamin D intake of 600 IU
  • Regular weight-bearing exercise
  • Smoking cessation

Return to Fertility

Return to fertility is delayed 9-18 months after discontinuation, making DMPA less suitable for women who may want to conceive within the next 1-2 years. 5, 6, 7

• Women with lower body weights conceive sooner than those with higher body weights 2
• Ovulation resumes when MPA levels fall below 0.1 ng/mL 3

Pre-Initiation Requirements

No examinations or laboratory tests are needed before initiating DMPA. 6

• No routine blood work required during ongoing use 6
• Baseline weight and BMI measurement may be useful for monitoring 6
• Bimanual examination and cervical inspection are NOT required (unlike IUDs) 4

Common Clinical Pitfalls

• Do not discontinue DMPA at 2 years based solely on duration concerns, as this increases risk of unintended pregnancy without clear benefit 6
• Do not routinely order bone density scans after 2 years unless other osteoporosis risk factors are present 6
• Do not order routine laboratory panels (CBC, metabolic panel, liver function tests) for DMPA monitoring, as this increases costs without evidence of benefit 6
• Counsel patients before initiation about menstrual changes and potential side effects to improve continuation rates 1, 6