Antacid does not reduce the bioavailability of oral contraceptive steroids in women.

The bioavailability of contraceptive steroids was studied in 12 women who were given an antacid and a contraceptive pill simultaneously. They were given a single pill containing ethinyl estradiol (EE2) 30 micrograms in combination with either norethisterone (NET) acetate 1 mg (n = 6), or levonorgestrel (LNg) 150 micrograms, (n = 6). Blood samples were collected up to 24 hours. Four weeks later the same pill was administered along with a single tablet of antacid (magnesium trisilicate 0.5 g and dried aluminum hydroxide 0.25 g) and blood samples were collected as before. Serum levels of NET, LNg and EE2 were measured by radioimmunoassay. No effect of antacid administration on bioavailability of any contraceptive steroid was observed as judged by peak levels and areas under concentration-time curve (AUC). Incidentally, significantly higher serum concentrations of EE2 were observed when it was administered in combination with NET than with LNg. The possible reasons for this finding are discussed.