Taking 5 tablets of tamoxifen 20 mg at the same time (total 100 mg) is significantly more than the usual prescribed daily dose, which is typically 20 mg once daily. This could increase the risk of side effects such as nausea, vomiting, dizziness, headache, hot flashes, blood clots, vision changes, or even more serious complications. If someone has taken this much tamoxifen at once, they should seek immediate medical advice or go to the nearest hospital emergency department. Always follow your doctor’s prescription carefully and never take more than the directed amount.