简单总结一下就是：多次吃也是安全的，不过通常有比紧急避孕药更安全和有效的避孕方法。

原文标题：

Fact sheet on the safety of levonorgestrel-alone emergency contraceptive pills (‎LNG ECPs)‎

关于左炔诺孕酮单一成分的紧急避孕药的安全性说明书

原文链接（网页中可下载pdf）

https://apps.who.int/iris/handle/10665/70210

该文部分内容摘录：While the World Health Organization (WHO) recommends
a single dose of levonorgestrel (1.5 mg) for emergency use within 120 hours after unprotected sex, repeat use does not pose any known health risks.4,5 Even among women who used
ECPs more than once in one menstrual cycle, no serious adverse outcomes were reported.6

世界卫生组织的该pdf文档参考文献里列入了另一篇来自“国际紧急避孕联盟（International Consortium for Emergency Contraception）”的文章：

Repeated Use of Emergency Contraceptive Pills: The Facts  链接https://www.cecinfo.org/publications-and-resources/icec-publications/repeated-use-emergency-contraceptive-pills-facts/

Emergency contraceptive pills (ECPs) can reduce the risk of pregnancy after unprotected sexual intercourse when a woman believes that her regular contraceptive method has failed, no method was used, or sex was forced. This fact sheet addresses both levonorgestrel (LNG) ECPs, which are the most commonly used EC method throughout the world, and ulipristal acetate (UPA) ECPs, for which less evidence exists.

The research shows that ECPs are extremely safe, even when used repeatedly. Compared with the potential health risks of pregnancy, taking ECPs to prevent unintended pregnancy is much safer. Women should be able to access and use ECPs as many times as they need. However, ongoing methods of contraception are more effective than ECPs. Only barrier methods, such as condoms, protect against HIV and sexually transmitted infections (STIs).

Are ECPs safe when used repeatedly?

Although ECPs are labeled for single use, use more than once even in the same menstrual cycle does not pose any known health risks. Repeat use of ECPs is classified as Level 1 in the World Health Organization’s Medical Eligibility Criteria (Level 1 indicates “a condition for which there is no restriction for the use of the contraceptive method”).1,2

ECPs pose no risk of harmful overdose and have no contraindications or major drug interactions.3,4 They can cause minor side effects, such as menstrual irregularities and nausea, which typically last only a short time.5 These effects are not medically harmful, and ultimately each woman should decide for herself whether they are acceptable for her.6

ECPs will not harm a fetus if a woman is already pregnant.7,8,9,10 Research also shows that LNG ECPs have no effect on future fertility.11 According to a meta-analysis of 136 studies, ECPs are not associated with an increased risk of ectopic pregnancy.12

While taking ECPs is extremely safe, pregnancy comes with known health risks; continuing a pregnancy and giving birth or resorting to unsafe abortion both present significant risks to women.

How effective are ECPs when used repeatedly?

On average, LNG ECPs reduce pregnancy by 59 to 95% for each individual act of intercourse; UPA ECPs reduce pregnancy by 85% and have been found to be comparatively more effective than LNG. The precise efficacy of ECPs depends mostly on the woman’s cycle day when ECPs are taken and how soon they are taken after unprotected sex.13,14,15,16 There is no evidence to suggest that ECPs become less effective when used repeatedly.

If a woman has unprotected sex more than once in the same menstrual cycle, should she take ECPs again?

ECPs provide contraceptive protection for only a short period of time. Women who have taken ECPs once in their cycle are still at risk of pregnancy later in that same cycle since ECPs can work by delaying ovulation.15,17 This means that if a woman has unprotected intercourse after taking EC, she is still at risk of pregnancy and should definitely consider taking ECPs again. Because both LNG and UPA remain in the body for some time after ingestion, ECPs do not need to be taken more than once every 24 hours if multiple acts of unprotected sex occur within this timeframe.18,19

Can women use ECPs as their regular or only contraceptive method?

Robust data are not yet available about the effectiveness of ECPs when used as a regular, ongoing contraceptive method, although several older studies suggest that such use of LNG ECPs is safe and may have effectiveness rates comparable to use of condoms.20 A recent study of regular UPA ECP use (either every 5 or 7 days) suggests that ovulation eventually occurred in most women; in other words, a woman may still be at risk of pregnancy.4

What do leading health organizations say about repeat use of ECPs?

The World Health Organization’s 2015 Medical Eligibility Criteria (MEC) states that “There are no restrictions on repeated use for COCs, LNG or UPA for ECPs (MEC Category 1).”1 (COCs are combined oral contraceptives, which in certain doses can be used as emergency contraception.) The American Congress of Obstetricians and Gynecologists (ACOG) states that “emergency contraception may be used more than once, even within the same menstrual cycle.”21

How common is repeated use of ECPs?

Demographic and Health Surveys (DHS) show that in many developing countries, fewer than 4% of women of reproductive age have ever used ECPs.22 From this population of ECP users, we have limited information about how many women have used them more than once; however, two studies,23,24 each conducted in Kenya and Nigeria, suggest that frequent use of ECPs is relatively uncommon.*

Recommendation

Medical research provides no basis for limiting the number of times that women use ECPs, even within the same cycle. ECPs are extremely safe; they are always safer than pregnancy. Women have many reasons for making different choices about contraception, and using ECPs is more effective than using no contraceptive method. While women should know that ECPs are less effective than ongoing contraceptive methods and do not protect against STIs, each woman can use ECPs every time she has unprotected sex and wants to avoid unwanted pregnancy.

（原文参考文献和脚注略去）