It should not be used in pregnancy, breastfeeding or children, but accidental usage in these situations is unlikely to be harmful. Use oestriol with caution in epilepsy, migraine, heart failure, high blood pressure, kidney disease, diabetes, porphyria or uterine disease, and not at all if suffering from liver disease, breast or genital cancer, blood clots, undiagnosed vaginal bleeding, endometriosis, porphyria or otosclerosis.
Side effects may include abnormal uterine bleeding, vaginal thrush, nausea, fluid retention, weight gain, breast tenderness. Unusual effects may include a rash, blurred vision, vomiting, bloating, intestinal cramps, pigmentation of skin on face. Severe but rare reactions may be blood clots, calf or chest pain and yellow skin (jaundice). Smoking increases the risk of serious side effects.