Diphenhydramine should be used cautiously in patients with renal impairment as it is metabolized by the liver and excreted primarily by the kidneys. Dosage adjustments may be necessary.

Patients with renal impairment require careful monitoring when prescribed diphenhydramine due to potential accumulation and increased risk of toxicity, including anticholinergic effects and CNS depression.

For those with renal impairment or chronic kidney disease, lower doses of diphenhydramine are advised, and renal function should be monitored to avoid toxicity from accumulation of the drug.