Recent drug studies have investigated the incidence of blood clots among patients using the Ortho Evra[R] birth control patch. In this article, the author describes an investigation of oestrogen levels in the body resulting from the application of the Ortho Evra[R] birth control patch versus daily use of Ortho-Cyclen[R] birth control pills. Prescribing information provided by the manufacturer and mathematical models from pharmacokinetics are used to investigate differences between the two birth control methods. Given the pharmacokinetic information on these drugs, applied calculus students are asked to recalculate the size of the patch so that the areas under the curves are equal. Their findings are compared to studies reported in the prescribing information produced by Ortho McNeil Pharmaceuticals, the manufacturers of these products.