No Clinically Significant Drug-Drug Interaction Between Nitrofurantoin and Acyclovir
There is no documented pharmacokinetic or pharmacodynamic drug-drug interaction between nitrofurantoin and acyclovir that would require dose adjustment or avoidance of concurrent use. These medications can be safely prescribed together when clinically indicated.
Evidence Base
The FDA drug labels for both nitrofurantoin and acyclovir do not list each other as interacting medications. 1, 2
A comprehensive 1996 review of antiviral drug interactions found no pharmacokinetic interaction between acyclovir and other commonly co-administered medications, and nitrofurantoin was not identified as a problematic combination. 3
A 1985 systematic review analyzing major reported reactions and interactions with nitrofurantoin from both published and unpublished sources did not identify acyclovir as an interacting agent. 4
The 2009 American Geriatrics Society consensus guidelines on renally cleared medications addressed both nitrofurantoin and acyclovir independently for dose adjustments in renal impairment, but did not flag any interaction between them. 5
Important Renal Considerations
While these drugs do not interact with each other, both require renal function assessment before prescribing because they are primarily renally cleared:
Nitrofurantoin should be avoided entirely when creatinine clearance is <30 mL/min due to inadequate urinary drug concentrations and increased risk of systemic toxicity (particularly pulmonary and neurologic adverse effects). 5, 6
Acyclovir requires dose reduction in renal impairment to prevent accumulation and toxicity; the half-life and total body clearance are dependent on renal function. 5, 2
Clinical Pitfalls to Avoid
Do not confuse acyclovir with valacyclovir: While acyclovir has no documented interaction with nitrofurantoin, valacyclovir (the prodrug of acyclovir) has been associated with increased acute kidney injury risk when combined with NSAIDs—though not with nitrofurantoin specifically. 7
Monitor for additive nephrotoxicity risk: Although not a direct drug interaction, both medications can contribute to renal complications in vulnerable patients (elderly, pre-existing renal disease, dehydration). Ensure adequate hydration and baseline renal function assessment. 2
Be aware of nitrofurantoin's documented interactions with other agents: Nitrofurantoin has established interactions with fluconazole (precipitating combined pulmonary and hepatic toxicity) and methotrexate (causing irreversible pulmonary fibrosis), but acyclovir is not among these problematic combinations. 8, 9