GHK-Cu and Diphenhydramine Interaction: What Clinicians and Patients Should Know

Is there a known interaction between GHK-Cu and diphenhydramine?

No FDA-approved drug contains GHK-Cu, so there is no FDA-approved GHK-Cu drug label that describes an interaction profile for the pair [1][2]. Diphenhydramine itself causes additive drowsiness with alcohol, sedatives and tranquilizers, and the injection label says MAO inhibitors can intensify its anticholinergic effects. In a study of 16 healthy men, diphenhydramine reduced metoprolol clearance in CYP2D6 extensive metabolizers; other CYP2D6 drugs were not tested [3][4][7].

What is known about GHK-Cu?

GHK-Cu is the copper complex of GHK, a tripeptide found in human plasma, saliva and urine, with levels reported to decline with age [8]. Its reported collagen, metalloproteinase and wound-healing actions come from laboratory and animal work, not proven human effects [8]. Topical copper peptide creams and serums are sold as cosmetics, which FDA does not approve before sale; injectable GHK-Cu is not a cosmetic and is not approved [2]. FDA states that compounded injectable drugs containing GHK-Cu may pose an immunogenicity risk from aggregation and peptide-related impurities, with limited human data to inform safety [5].

The human trials found are small. In a randomized trial of 13 patients after CO2 laser resurfacing, objective measures found no significant difference in redness, wrinkles or overall skin quality, while a patient questionnaire favored GHK-Cu (P=.04) [9]. In an 86-patient venous ulcer trial, a 0.4% tripeptide copper complex cream was no different from an inert vehicle for ulcer size [10]. A 2026 systematic review calls the clinical literature thin and fragmented [11].

What diphenhydramine effects matter?

Diphenhydramine is a first-generation H1 antihistamine with anticholinergic and sedative effects that also inhibits CYP2D6 [4][7]. The Benadryl Allergy tablet label lists 25 to 50 mg every 4 to 6 hours for adults and children 12 and over, with no more than 6 doses in 24 hours. It warns that marked drowsiness may occur, that alcohol, sedatives and tranquilizers may increase drowsiness, and to be careful when driving or operating machinery [3]. The label also says to ask a doctor before use with glaucoma, a breathing problem such as emphysema or chronic bronchitis, or trouble urinating due to an enlarged prostate [3]. FDA has warned that doses above the label can cause serious heart problems, seizures, coma or death [12].

The 2023 AGS Beers Criteria advise avoiding first-generation antihistamines in adults 65 and older (moderate quality of evidence, strong recommendation) because they are highly anticholinergic, with risks of confusion, dry mouth and constipation. Cumulative exposure is associated with falls, delirium and dementia [6].