Category B—Both animal & published human experience suggest that diphenhydramine is safe for use in human pregnancy. At least one review has concluded that diphenhydramine is the drug of choice if parenteral antihistamines are indicated in pregnancy.
Diphenhydramine is excreted into human breast milk, but levels have not been reported. Although the levels are not thought to be sufficiently high to affect the infant after therapeutic doses, the manufacturer considers the drug contraindicated in nursing mothers. The reason given for this is the increased sensitivity of newborn or premature infants to antihistamines.
Avoid as its more likely to cause anticholinergic side effects in elderly patients AND more likely to accumulate d/t its longer t½ in the elderly (13.5°)
No dosage adjustment necessary.
Dosage reduction may be warranted for patients with hepatic impairment; diphenhydramine is extensively metabolized in the liver.