No contraceptive is utterly foolproof. Any contraceptive can fail. So IUD and contraceptive pill are no different. IUD is very effective. Only one in hundred pregnancies can happen when it is properly used. That means it is properly placed in the uterus and replaced at appropriate time. OC pills when it is properly used the chances of pregnancies is only one in thousand. But when it is little casually used the chances of failure is one in hundred. So the circumstances like vomiting and diarrhoea need to be remembered when taking OC pills. That can make the OC pills fail. Also when you are taking medications like antibiotics like rifampicin, rifabutin, they can interfere with the absorption of these OC pills and get ineffective. So any contraceptive method can fail and you should be cautious regarding that.
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