What are the potential interactions between famotidine (Famotidine) and Bactrim (Sulfamethoxazole/Trimethoprim) in patients?

Famotidine and Bactrim (Sulfamethoxazole/Trimethoprim) Interaction

There is no clinically significant interaction between famotidine and Bactrim (sulfamethoxazole/trimethoprim) that would require dose adjustments or contraindicate their concomitant use in most patients.

Pharmacological Basis

Famotidine Metabolism

• Famotidine is an H2-receptor antagonist primarily eliminated through renal excretion
• Unlike cimetidine, famotidine does not bind to or inhibit the cytochrome P-450 system 1
• Approximately 70% of famotidine is excreted through the kidney, mainly by tubular secretion 2

Bactrim Metabolism

• Trimethoprim/sulfamethoxazole (Bactrim) is primarily excreted by the kidneys 3
• Metabolites of both trimethoprim and sulfamethoxazole are excreted in the urine 3

Lack of Significant Drug-Drug Interaction

The evidence suggests no significant pharmacokinetic interaction between these medications because:

1. Famotidine does not interact with the cytochrome P-450 system, unlike other H2-receptor antagonists such as cimetidine 1
2. While cimetidine inhibits multiple CYP enzymes (1A2, 2C9, 2C19, 2D6, 2E1, and 3A4), famotidine has no significant effect on these enzymes 1
3. No specific interaction between famotidine and Bactrim is mentioned in any of the guidelines reviewed

Special Considerations

Renal Impairment

Both medications require dose adjustments in renal impairment, though not because of interaction:

• Famotidine:

  • Normal dose for creatinine clearance >60 ml/min
  • Reduce dose by 50% when creatinine clearance is 30-60 ml/min
  • Reduce dose by 75% when creatinine clearance is <30 ml/min 2, 4

• Bactrim:

  • Dose adjustment recommended when creatinine clearance is <30 ml/min 3

Potential Concerns with Other Medications

While famotidine and Bactrim don't interact significantly with each other, be aware of other potential interactions:

• Bactrim has significant interactions with:
  • Anticoagulants (potentiates warfarin activity)
  • Methotrexate (increases methotrexate levels)
  • Phenytoin (reduces metabolism)
  • Several other medications 1

Monitoring Recommendations

When using these medications concomitantly:

1. Monitor renal function, especially in elderly patients or those with pre-existing renal impairment
2. Be vigilant for side effects of either medication, though these are not expected to be enhanced by co-administration
3. If the patient is also taking methotrexate, be aware that Bactrim can significantly increase methotrexate levels 1

Conclusion

The available evidence does not suggest any clinically significant interaction between famotidine and Bactrim that would require special precautions beyond the standard monitoring recommended for each medication individually.