Levonorgestrel 1.5 mg tablets for emergen cycontraception works best when you take it as soon as possible after unprotected sex. It is effective within 72 hours (3 days) of unprotected sex. Levonorgestrel 1.5 mg tablets is not effective as regular long-term method of contraception and will not always stop a pregnancy (not 100% effective). It should not replace regular, longterm contraception.