左炔諾孕酮（Levonorgestrel）
Overview
Levonorgestrel is a progestin used primarily as an emergency contraceptive to prevent pregnancy after unprotected sexual intercourse or contraceptive failure.
Mechanism of Action
It primarily works by delaying or inhibiting ovulation. It may also alter the cervical mucus and the endometrium, making sperm penetration and implantation more difficult.
Dosage and Administration
For emergency contraception, a single dose of 1.5 mg should be taken as soon as possible, ideally within 72 hours (3 days) after unprotected sex. Efficacy decreases with time.
Side Effects
Common side effects include nausea, vomiting, fatigue, headache, dizziness, and changes in the menstrual cycle (early or delayed bleeding).
Contraindications
It should not be used if pregnancy is already established. It is contraindicated in patients with severe hepatic impairment.
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