In studies with oral contraceptives (birth control pills) containing estrogens, cigarette smoking during the use of estrogens was shown to cause an increased risk of serious side effects affecting the heart or blood circulation, such as dangerous blood clots, heart attack, or stroke. The risk increased as the amount of smoking and the age of the smoker increased. Women aged 35 and over were at greatest risk when they smoked while using oral contraceptives containing estrogens. It is not known if this risk exists with the use of androgens and estrogens for symptoms of menopause. However, smoking may make estrogens less effective.