When to take birth control pills to have contraceptive effect depends on which kind of birth control pills, the common short-acting contraceptive pills, emergency contraceptive pills, each kind of drug is different. Common medications for short-acting contraceptives include compound norethindrone tablets and compound progesterone tablets. Among them, compound norethindrone tablets need to be taken on the 5th day of the menstrual cycle, one tablet a day for 22 days without interruption, and then again after the next menstrual period, according to this method. Compound progesterone tablets should be taken on the 1st day of the menstrual cycle, one tablet a day for 21 days, without interruption, and then start to take it again after 7 days of stopping, in the same way as above. Emergency contraceptive pills are common levonorgestrel capsules, mifepristone capsules, etc., levonorgestrel capsules to be taken within 72 hours after sex, can be taken two at a time, but also in the 72 hours to take the first capsule, every 12 hours and then take the second capsule. Mifepristone capsules are to be taken 2 capsules 72 hours after sex and need to be taken orally on an empty stomach or 2 hours after a meal. Regardless of the type of birth control pill, you should take it under the supervision of your doctor and not use it on your own. You can consult your doctor first and choose the right medication for your situation.