Factors influencing the reliability of oral contraceptives

Abstract

The reliability of oral contraceptives (OC) is high but several known factors can potentially induce failures. The dose of estrogen (EE) has been reduced through the years. Interaction with concomitant use of other medicine is wellknown and gastroenteritis can reduce the uptake of EE. To further characterize and quantify these factors we have performed this study. Among patients admitted for legal abortion we selected those who had taken OC (only combined preparations; gestagen-only preparations were excluded) according to the prescription but nevertheless became pregnant. Various parameters were noted. Among 8058 women 70 women were found. Twenty-nine used three-phased and 25 used low-dose OC while the rest used high-dose, two-phased or unknown OC (four, five and seven patients respectively). Five patients used other medicine concurrently (salbutamol, astemizol, mianserin, chlorcyclizin, paradryl, carbamazepin, lithium, chlorprotoxin and imipramin). Sixteen patients had symptoms of gastroenteritis at the time of conception. Forty-nine patients had failure without any known influencing factors. There was no significant difference between the various OC used by the patients compared to sales of them in Denmark. About two-thirds of the patients wanted to continue with OC as future prevention. Failure of OC is a rare event but can occur in case of concurrent gastroenteritis or use of other medicine but in many cases, however, no definite cause can be determined. The EE-content of the various OC was not found to have any influence. Although OC-failure had occurred two-thirds of the patients wanted to continue the use of OC.