Summary Safety Review - Methimazole - Assessing the Potential Risk of Inflammation of the Blood Vessels (Vasculitis)

Review decision

A Summary Safety Review complements other safety related information to help Canadians make informed decisions about their use of health products. Each summary outlines what was assessed in Health Canada’s review, what was found and what action was taken by Health Canada, if any.


Issued: 2020-04-21

Product

Tapazole (methimazole) and generics

Potential Safety Issue

Inflammation of the blood vessels (vasculitis)

Key Messages

  • Methimazole is a prescription drug authorized for sale in Canada to treat conditions where too much thyroid hormone is produced.
  • Health Canada reviewed the potential risk of inflammation of blood vessels (vasculitis) with the use of methimazole. The review was triggered by updates made by the United States Food and Drug Administration (US FDA) to the product safety information for methimazole related to this risk.
  • Health Canada's review of the available information concluded that there is a link between the risk of vasculitis and the use of methimazole.
  • Health Canada is working with manufacturers to update the safety information of all methimazole products to include information about this potential risk.

Overview

Health Canada reviewed the potential risk of vasculitis with the use of methimazole. Vasculitis is an inflammation of blood vessels that may prevent blood from reaching organs in the body. The safety review was initiated when Health Canada became aware that US FDA updated the product safety information for methimazole with new information on the risk of vasculitis.

Use in Canada

  • Methimazole is a prescription drug authorized for sale in Canada to:
    • treat hyperthyroidism, a condition in which too much thyroid hormone is produced by the thyroid gland.
    • treat and prepare the overactive thyroid gland for surgical removal or for radioactive iodine treatment.
    • treat the overactive thyroid gland that cannot be removed.
  • Methimazole has been marketed in Canada since 1951 under the brand name Tapazole and is currently available as 5 mg and 10 mg oral tablets. A generic version of Tapazole is also available for sale in Canada.
  • The average use of methimazole oral tablets in Canada over the last 5 years was approximately 200,000 prescriptions annually.

Safety Review Findings

  • Health Canada reviewed 13 international case reports of vasculitis in patients receiving methimazole. Of the 13 case reports, 11 reports showed a possible link to methimazole use, one report showed a slightly stronger link and one report was unlikely linked to methimazole use.
  • Health Canada also assessed 22 articles from the published scientific literature. Many of them suggested that there was a potential risk of vasculitis with methimazole use but the frequency of the risk was considered very rare (less than one person suffered from vasculitis out of 10,000 taking methimazole).

Conclusions and actions

  • Health Canada's review concluded that there is a link between the risk of vasculitis and use of methimazole.
  • Health Canada is working with the manufacturers to update the Canadian safety information on methimazole products to inform about this risk.
  • Health Canada will continue to monitor safety information involving methimazole products, as it does for all health products on the Canadian market, to identify and assess potential harms. Health Canada will take appropriate and timely action if and when any new health risks are identified.

Additional information

The analysis that contributed to this safety review included scientific and medical literature, Canadian and international adverse reaction reports and what is known about the use of this drug both in Canada and internationally.

For additional information, contact the Marketed Health Products Directorate.

Report a problem or mistake on this page
Please select all that apply:
Date modified: