左炔诺孕酮（计划B® 或同等学名药）是一种仅含孕激素的紧急避孕药，用于在发生性行为 后防止怀孕。片剂（丸剂）应尽快服用，最好在最后一次性接触后24 小时内服用。它在发 生性行为后5 天（120 小时）内有效，但随着时间的推移，效果会降低。
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