Started taking methimazole last month. used to have heavy periods every 28 days 5-7 days. after 3 weeks on methimazole, i have had bleeding like a period or spotting continuously for the past 3 weeks. i am 49. could the medication be the cause?

Methimazole is generally not a cause for the bleeding described, but at age "50", it is common for increased estrogen production to cause thickened endometrium, which will break down and cause bleeding. During this time prior to true menopause, ask your GYN physician about low dose oral contraceptives, which stop ovarian hormone production, and other guidance for the bleeding.