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Combined oral contraceptive pill (COCP) is absolutely safe for her. It does not cause cancer (low dose COCP). Just check the content of COCP which she is taking. If dose of Estrogen (Ethinyl estradiol) is 20 or 30 mcg, she can safely continue the medicine. In fact, COCP in such a case prevents endometrial cancer due to its progesterone content. Regarding missed period, it normally does not happen with COCP. Normally period starts between 3 and 15 days of stopping the last tablet in the pack. If it has been more than 15 days since last tablet, consult a gynecologist. If it has been less than 15 days, she can wait till 15 days from last pill for periods to come. In any case, she cannot start next pack without having periods unless instructed by doctor after examination. Do not forget to tell her to use another form of contraception like condom if she is sexually active.