Final answer:
Oral contraceptives inhibit ovulation and increase blood clot risk, which can interact negatively with phenobarbital, reducing contraceptive efficacy.
Explanation:
Oral contraceptives work by inhibiting GnRH secretion, which prevents ovulation. They can lead to an increased tendency for blood clots due to estrogen's effect on enhancing blood clotting. Phenobarbital, a barbiturate, can interact with oral contraceptives and decrease their effectiveness, potentially leading to unintended pregnancies.
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