No Clinically Significant Interaction Between Lithium and Nitrofurantoin
Nitrofurantoin does not alter lithium serum concentrations and can be safely used concurrently. There is no documented pharmacokinetic or pharmacodynamic interaction between these two medications.
Evidence Base
The available evidence does not identify nitrofurantoin as an interacting medication with lithium. When examining drug interactions that affect lithium levels, the primary concern involves medications that alter renal function or electrolyte balance, as lithium is eliminated almost exclusively through the kidneys 1.
Medications That DO Interact with Lithium
The well-established lithium interactions include 2, 3:
- NSAIDs (can increase lithium levels by 25-40%)
- Thiazide diuretics (significant risk of toxicity)
- ACE inhibitors and angiotensin receptor antagonists (impair lithium elimination)
- Loop diuretics (variable minor effects)
A comprehensive study examining drug-drug interactions as determinants of elevated lithium levels specifically investigated NSAIDs, diuretics, renin-angiotensin inhibitors, theophylline, and antibiotics as potential culprits 4. While antibiotics as a class showed some association (OR 2.70), nitrofurantoin was not specifically implicated in causing lithium toxicity.
Why Nitrofurantoin is Safe with Lithium
Nitrofurantoin's pharmacokinetic profile explains the lack of interaction:
- Achieves high urinary concentrations with minimal systemic absorption 5, 6
- Does not significantly affect renal function in the way that NSAIDs or diuretics do
- Does not alter electrolyte balance or glomerular filtration rates
- Minimal tissue and serum concentrations limit systemic drug interactions 5
Clinical Recommendations
When prescribing nitrofurantoin to patients on lithium:
- No lithium dose adjustment is necessary
- Standard lithium monitoring protocols remain unchanged
- Use nitrofurantoin at standard doses (100 mg twice daily for 5 days for uncomplicated cystitis) 7, 8
- Monitor for nitrofurantoin-specific adverse effects (pulmonary toxicity, peripheral neuropathy) per usual practice 9
Important Caveat
While nitrofurantoin itself does not interact with lithium, if a patient develops significant gastrointestinal side effects (nausea, vomiting, diarrhea) from nitrofurantoin 9, this could theoretically lead to dehydration and secondary lithium retention. However, this is a general concern with any medication causing fluid loss, not a specific drug-drug interaction.
Bottom line: Nitrofurantoin remains an appropriate first-line choice for urinary tract infections in patients taking lithium 7, 8, 10, with no need for special precautions beyond standard monitoring.