text: Serious haematological disorders and porphyria (due to the sulfonamide component of the preparation).
  Severe chronic kidney disease (CrCl <15 ml/min) where repeated measurements of the plasma concentration cannot be performed
  Interactions
  Its use is advised against in people being concomitantly treated with:
  ACE inhibitors like captopril, enalapril, lisinopril, perindopril, and ramipril due to the potential for additive hyperkalaemic effects
  Prilocaine — additive risk of methaemoglobinaemia
  Antiarrhythmics like amiodarone (increased risk of ventricular arrhythmias) and dofetilide (increased risk of QT interval prolongation)
  Antibacterials like dapsone (increases plasma levels of both drugs), methenamine (increased risk of crystalluria) and rifampicin (as it may lead to an increased plasma level of rifampicin and lower plasma levels of trimethoprim)
  Anticoagulants like warfarin and acenocoumarol — anticoagulant effects of either drug is potentiated by this combination
  Sulfonylureas — effects enhanced
  Phenytoin, half-life of phenytoin is increased
  Antifolates like pyrimethamine, proguanil and methotrexate increase the risk of associated side effects like bone marrow toxicity, folic acid supplementation should be considered. A significant risk of megaloblastic anaemia exists with doses of pyrimethamine in excess of 25 mg/wk.
  Antivirals, more specifically, lamivudine (increased plasma concentrations of lamivudine), zalcitabine (increased plasma concentrations of zalcitabine) and zidovudine (increased risk of haematological reactions)