Effect of low-dose oral contraceptives on natural killer cell activity

Several reports indicate an association between oral contraceptives and increased infection rates. One mechanism that could explain this increased infection rate is a decrease in immune function. A study comparing T cell subsets showed no differences in numbers between oral contraceptive users and controls. In this study, natural killer cell activity was compared in women before and 3 and 6 months after oral contraceptive use. There was a statistically significant decrease in NK cell activity after three months. There was no further decrease by six months and the differences were no longer significant due to greater variability. No infections were reported during the study period. Thus, the observed reduction in NK activity was either physiologically insignificant or the previously reported increase in infections may be the result of non-immunological factors.