Understanding contraindications for sulfamethoxazole-trimethoprim is critical due to the relatively common occurrence of sulfa allergies and important drug interactions. Careful screening helps prevent serious adverse reactions and treatment failures.
Key contraindications include documented sulfa allergies, age under 2 months, severe renal impairment, folate deficiency anemia, and significant drug interactions with methotrexate and potassium-raising medications. The dual mechanism requires consideration of contraindications for both components.
History of hypersensitivity reactions to any sulfonamide medication
Risk of kernicterus from bilirubin displacement
Trimethoprim further inhibits folate metabolism
Use is contraindicated in children with marked hepatic damage or significant renal impairment unless closely monitored and dose-adjusted. Impaired clearance can lead to drug accumulation and increased toxicity. Monitoring of renal function and electrolytes is important, especially during prolonged therapy.
SMX-TMP interferes with folic acid metabolism, so it should not be used in children with documented folate deficiency–related anemias unless under close hematologic supervision. In these patients, alternative antibiotics may be safer.
Avoid concurrent use with drugs like methotrexate or phenytoin without specialist guidance, as SMX-TMP can displace these agents from protein-binding sites and interfere with metabolism, increasing toxicity. Also use caution in children taking ACE inhibitors or potassium-sparing diuretics, as the combination may increase the risk of hyperkalemia.
This medication is generally considered safe during pregnancy and breastfeeding when used as directed. However, if you're pregnant or breastfeeding and need to give this medication to your child, it's always a good idea to discuss this with your healthcare provider to ensure it's the best choice for your specific situation.
Specific drugs: See description
Effect: Severe bone marrow suppression, mucositis
How it happens: Displacement from protein binding and decreased renal clearance
Management: Avoid combination or monitor with extreme caution
Specific drugs: See description
Effect: Increased bleeding risk, elevated INR
How it happens: Inhibition of warfarin metabolism, displacement from proteins
Management: Monitor INR closely, adjust warfarin dose
Specific drugs: See description
Effect: Risk of dangerous hyperkalemia
How it happens: Both drugs can increase potassium levels
Management: Monitor potassium levels, especially with renal impairment
Specific drugs: See description
Effect: Phenytoin toxicity: ataxia, nystagmus, confusion
How it happens: Inhibition of phenytoin metabolism
Management: Monitor phenytoin levels, adjust dose if needed
Specific drugs: See description
Effect: Digoxin toxicity: nausea, arrhythmias
How it happens: Reduced gut bacteria that metabolize digoxin
Management: Monitor digoxin levels in susceptible patients
Screening question
If yes: Consult healthcare provider
Screening question
If yes: Consult healthcare provider
Screening question
If yes: Consult healthcare provider
Screening question
If yes: Consult healthcare provider
Screening question
If yes: Consult healthcare provider
Screening question
If yes: Consult healthcare provider
Frequency
Baseline and weekly for prolonged therapy
Action Thresholds:
Frequency
Baseline and as indicated
Action Thresholds:
Frequency
If risk factors present
Action Thresholds:
Frequency
Ongoing during therapy
True sulfa allergy prevalence is lower than reported
Hydration is therapeutic, not just supportive
Cross-reactivity with non-antibiotic sulfonamides is rare
Trimethoprim component can cause isolated hyperkalemia