Methenamine
Adverse Effect: increased risk of crystalluria Clinical Management: Avoid concurrent administration of methenamine and sulfamethizole, since sulfamethizole may form an insoluble precipitate in acidic urine. If a sulfonamide must be used, sulfisoxazole, which is more soluble, would be an alternative.
Phenytoin
Adverse Effect: An increased risk of phenytoin toxicity (ataxia, hyperreflexia, nystagmus, tremor) Clinical Management: Consider monitoring serum phenytoin levels five days after initiating or stopping either sulfadiazine or sulfamethizole; decreased phenytoin dosage may be required with concomitant therapy. |