Caffeine is widely consumed by both men and women but its pharmacological effects have not been well studied in the area of assisted reproduction. Various authorities indicate that caffeine consumption up to 400 mg per day is safe, but caution is advised for women who are, or are contemplating pregnancy and they should keep their intake under 200 mg per day. Our lifestyle studies at Curtin University showed adverse effects from caffeine consumption in both men and women. For men, fertilization rates were negatively associated with caffeine consumption (P<0.05) as well as IVF-related stress (P<0.005). Furthermore, caffeine negated the beneficial effect of male alcohol consumption, comprising mainly beer, on fertilization. For women, fertilization rates were negatively associated with caffeine consumption (P<0.005) and smoking history in years (P<0.001). However, our studies could not show any effect of caffeine consumption on the chance of pregnancy or miscarriage up to week 12 from either male intake (up to 4495 mg per week) or female intake (up to 2706 mg per week). We conclude that there is likely to be an effect from caffeine on one-Carbon metabolism and future studies need to interrogate the concomitant nutritional intake of B-vitamins and serum homocysteine levels, which can indirectly indicate deficiency or interference with this important metabolic pathway.