I was wondering if Benadryl can be harmful to the kidneys? I take it a few times a week for allergies or to help me sleep.
Although there have been a few case reports of Benadryl (diphenhydramine) induced kidney damage, they have only been in cases of overdose and in those with certain pre-existing conditions. It is generally considered to be safe and non-damaging to the kidneys.
In fact, most literature suggests that no dosage adjustments are required in those with reduced kidney function or impairment.
Benadryl (diphenhydramine) is a first-generation, sedating antihistamine of the ethanolamine class. It is available in a variety of dosage forms, including:
While Benadryl is excreted renally (i.e. via the kidneys), it is extensively metabolized in the liver and metabolism speed may be reduced in the presence of hepatic impairment. Dosage reduction may be required in these individuals.
In terms of Benadryl damaging the kidneys, there have been a few case reports, but have typically only occurred in cases of overdose.
One such case study involved an individual who intentionally overdosed on Benadryl and alcohol, and was diagnosed with acute renal failure and subsequently suffered from rhabdomyolysis.
Another study noted that Benadryl, or any drug that has significant anticholinergic properties, can cause urinary retention and therefore increase the risk of renal kidney injuries in those with a pre-existing disease:
"Anticholinergic medications, such as diphenhydramine [Benadryl], can cause postrenal obstruction (blockage of the urinary tract). Obstruction can occur from the urinary tubule to the urethra, resulting in urine accumulation and ultimately increasing upstream pressure and decreasing GFR."
Since Benadryl can cause urinary retention (especially in the elderly or in those with pre-existing kidney disease), it is recommended to be used cautiously in individuals with bladder obstruction or urinary retention. This is a general precaution with any drug that has anticholinergic effects.