Carbimazole is a pro-drug converted to the active metabolite thiamazole. Thiamazole and methimazole are the same chemical. Thiamazole is the international nonproprietary name (INN) while methimazole is the United States adopted name (USAN).
Thiamazole is a thioamide used to treat hyperthyroidism such as in Graves’ disease. In the USA, thiamazole (known there as methimazole) is the most widely used thioamide. However, in many other parts of the world carbimazole is the preferred thioamide. The half-life of carbimazole is 3 to 6 hours, while the half-life of thiamazole is 6 to 15 hours. As thiamazole is the active compound, when a patient takes carbimazole, the half-life of the drug effect is 9 to 21 hours. Thus carbimazole can be taken once per day while thiamazole usually has to be taken three times per day or twice per day.