Side effects of carbimazole / propylthiouracil

Both drugs have similar side effects.

• Agranulocytosis is dangerous and affects about 0.3 in 1000 patients (1). It is not predicted from routine WBC measurements and should be suspected in patients on high doses presenting with a sore throat, mouth ulcer or any serious infection. The risk of agranulocytosis is lower with carbimazole than with propylthiouracil. Agranulocytosis is an absolute contraindication to further treatment with antithyroid drugs and treatment with radio-iodine should be given
  
  
• An itchy maculopapular skin rash occurs in 1-5% if cases.

Less common side effects may include:

• arthralgia
• nausea and vomiting
• fever
• jaundice
• vomiting
• alopecia
• carbimazole - congenital malformations reported associated with use of carbimazole include aplasia cutis congenita (absence of a portion of skin, often localised on the head), craniofacial malformations (choanal atresia; facial dysmorphism), defects of the abdominal wall and gastrointestinal tract (exomphalos, oesophageal atresia, omphalo-mesenteric duct anomaly), and ventricular septal defect (2)

Reference:

• Current Problems in Pharmacovigilance (February 1999), 25, 1-4.
• MRHA (February 2019). Drug Safety Update 12(7):1-8