Sex differences in pharmacokinetics causes medicine to be metabolized differently in women, and leads to women experiencing more side effects. This study aims to investigate whether the activity of enzyme CYP1A2 fluctuates between the early follicular phase and around ovulation. Caffeine and it’s main metabolite paraxanthine, was used as a marker for CYP1A2 activity measured by ratio(paraxanthine/caffeine). Urine samples were collected from six test subjects (age 16-19), and analysed using HPLC-MS. The results of the study suggest a possible link between women’s menstrual cycle and variation in CYP1A2 activity. It highlights the need for further studies with larger groups of participants, to better understand the pharmacokinetic differences between men and women.