A 34-year-old patient with bronchitis is planning to start erythromycin treatment (Ery-Max) 250 mg

Fråga: A 34-year-old patient with bronchitis is planning to start erythromycin treatment (Ery-Max) 250 mg x2x2. Is there any interaction between erythromycin and the contraceptive drug Exlutena (lynestrenol 0.5 mg)?

Sammanfattning: There has been only one case report of oral contraceptive failure in a woman taking erythromycin. The risk of becoming pregnant during oral contraceptive and erythromycin treatment is hard to judge but based on an extensive literature search, this risk is hardly increased.

Svar: Interactions between antibiotics and contraceptives have been reviewed previously in Drugline (1). In summary, risks for unexpected pregnancy during oral contraceptive intake increased strongly with rifampicin therapy, and probably also with griseofulvin treatment. Pregnancies in women receiving ampicillin or tetracycline with contraceptive preparations have also been described. There was only one case of oral contraceptive failure in a woman taking erythromycin reported to the British Committee on the Safety of Medicine (2). The Swedish Adverse Drug Reactions Advisory Committee has not received any reports of pregnancy in women who take contraceptives and erythromycin, although several pregnancies have been reported in women taking contraceptives together with rifampicin, phenytoin and carbamazepine. In contrast to these antibiotics, erythromycin is known to interact with a number of drugs by inhibition of their metabolism (3).

Mechanisms for the interaction between antibiotics and contraceptives include the enzyme induction effect of for example rifampicin, which increases the metabolism of contraceptives and reduces their plasma concentrations. It has also been suggested that broad-spectrum antibiotics, which kill the bacteria that hydrolyse the conjugates of contraceptives, prevent the enterohepatic recirculation of ethinyloestradiol and produce a fall in its plasma concentration. It has therefore been suggested that alternative contraceptive precautions be used by all women receiving broad-spectrum antibiotics and oral contraceptives (4).