What is the Difference Between Carbimazole and Methimazole?

Carbimazole and methimazole are both antithyroid drugs used to treat hyperthyroidism, such as Graves' disease. They have some differences in their molecular structure and the way they are metabolized in the body. Key differences between carbimazole and methimazole include:

1. Molecular structure: Carbimazole is a pro-drug that is converted to the active metabolite thiamazole in the body. Thiamazole and methimazole are the same chemical, but thiamazole is the international nonproprietary name (INN) while methimazole is the United States name.
2. Dosage equivalence: Carbimazole has a higher molecular weight than methimazole, so 5 mg of carbimazole is equivalent to 3 mg of methimazole. The starting dose for carbimazole is 5 mg every 8 to 12 hours.
3. Metabolism: Carbimazole is nearly totally converted to methimazole after oral administration to humans and cats. 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. This allows carbimazole to be taken once per day, while thiamazole usually has to be taken three times per day.
4. Availability: In the United States, methimazole (known there as thiamazole) is the most widely used thioamide. However, in many other parts of the world, carbimazole is the preferred thioamide.

Comparative Table: Carbimazole vs Methimazole

Carbimazole and methimazole are both antithyroid drugs used to treat hyperthyroidism, but they have some differences in their forms and side effects. Here is a table comparing the two:

Carbimazole is an inactive form of the drug, which is converted into its active form, methimazole, inside the body when taken for the treatment of hypothyroidism. Carbimazole has been preferred in some patients because, compared with methimazole, it may have fewer side effects, such as less frequent gastrointestinal problems. However, both drugs are effective in treating hyperthyroidism and have similar side effect profiles.