1. Clinical Overview
Medroxyprogesterone acetate (MPA) is a long-acting, synthetic progestin structurally related to progesterone. In the 150mg/ml concentration, it is formulated as a sterile aqueous suspension for deep intramuscular injection, primarily used for long-term contraception and endometrial protection. It exerts its effects by inhibiting the secretion of gonadotropins, preventing follicular maturation and ovulation, and inducing endometrial atrophy.
| Onset | Duration | Bioavailability |
|---|---|---|
| Contraceptive effect begins within 24 hours if administered within the first 5 days of a normal menstrual cycle. Full suppression of ovulation typically occurs within 2-4 weeks. | Contraceptive effect lasts for a minimum of 12 weeks (3 months) per 150mg injection. The return to fertility is delayed, with ovulation typically resuming within 6-12 months after the last injection, but can take up to 18 months. | Approximately 70-80% following intramuscular administration. |
2. Mechanism of Action
MPA is a potent synthetic progestin that exerts its primary contraceptive effect by suppressing the midcycle surge of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary. This inhibition of gonadotropin secretion prevents follicular maturation and ovulation. It also thickens cervical mucus (making it hostile to sperm penetration) and induces endometrial atrophy, creating an unfavorable environment for implantation.
3. Indications & Uses
- Long-term reversible contraception (injection every 3 months)
- Endometrial protection in women on estrogen replacement therapy (as part of HRT)
4. Dosage & Administration
Adult Dosage: 150 mg (1 ml) by deep intramuscular injection into the gluteal or deltoid muscle every 3 months (12-14 weeks). For contraception: First dose must be given within first 5 days of a normal menstrual cycle or immediately postpartum (if not breastfeeding).
Administration: Shake the vial vigorously immediately before use to ensure uniform suspension. Administer via deep IM injection ONLY. Do not administer intravenously. Use a 21- or 22-gauge, 1.5-inch needle. Aspirate before injection to avoid intravascular administration. Rotate injection sites.
5. Side Effects
Common side effects may include:
- Menstrual irregularities (spotting, breakthrough bleeding, amenorrhea)
- Weight gain (2-4 kg in first year)
- Headache
- Abdominal pain/discomfort
- Dizziness
- Fatigue
- Decreased libido
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Aminoglutethimide | Markedly decreases MPA levels, reducing efficacy. | Major |
| Strong CYP3A4 Inducers (e.g., Rifampicin, Carbamazepine, Phenytoin, St. John's Wort) | Increase metabolism of MPA, potentially reducing contraceptive efficacy. Alternative contraception advised. | Major |
| Anticoagulants (Warfarin) | MPA may decrease anticoagulant effect; monitor INR closely. | Moderate |
| Insulin/Oral Hypoglycemics | MPA may decrease glucose tolerance, requiring dose adjustment. | Moderate |
| Corticosteroids (e.g., Prednisolone) | Additive glucocorticoid effect; monitor for Cushingoid symptoms. | Moderate |
| Troleandomycin, Ketoconazole (CYP3A4 inhibitors) | May increase MPA levels. | Moderate |
7. Patient Counselling
- DO return for your next injection every 12-14 weeks as scheduled for continuous protection.
- DO inform all your doctors and surgeons that you are using this injection.
- DO use a non-hormonal backup contraceptive (like condoms) for the first 7 days after your first injection if it was not given within the first 5 days of your period.
- DONT massage the injection site vigorously after administration.
- DONT stop the injections without consulting your doctor; fertility may be delayed upon discontinuation.
8. Toxicology & Storage
Overdose: Acute overdose is unlikely due to route of administration. Theoretical symptoms from excessive systemic exposure could include nausea, vomiting, drowsiness, and withdrawal bleeding in females. Chronic overexposure (more frequent dosing) may lead to Cushingoid symptoms, severe fluid retention, hypertension, and profound amenorrhea.
Storage: Store below 25°C. Do not freeze. Protect from light. Keep the vial in the outer carton. The suspension is stable for 28 days after first puncture if stored correctly. Shake vigorously before use. Do not use if particulate matter or discoloration is observed.