Irregular cycles and reduced reliability of oral contraceptives may occur when these preparations are used concomitantly with drugs such as anticonvulsants, barbiturates, antibiotics, (e.g. tetracyclines, ampicillin, rifampicin, etc.), griseofulvin, the herbal remedy St Johns Wort, activated charcoal and certain laxatives.
Special consideration should be given to patients being treated with antibiotics for acne. They should be advised to use a non-hormonal method of contraception, or to use an oral contraceptive containing a progestogen showing minimal androgenicity, which have been reported as helping to improve acne without using an antibiotic.
Oral contraceptives may diminish glucose tolerance and increase the need for insulin or other antidiabetic drugs in diabetics.