AOD-9604 and Diphenhydramine Interaction: What Patients and Clinicians Need to Know

Is there a documented interaction between AOD-9604 and diphenhydramine?

No. AOD-9604 has no FDA-approved label or approved dose, so there is no labeled interaction information to draw on [1][2]. Anything said about the combination has to be assembled from each product separately.

What is AOD-9604?

AOD-9604 is a 16-amino-acid fragment of human growth hormone: hGH residues 177 to 191 with an extra tyrosine at the N-terminus [5]. An Australian company developed it as an obesity candidate; phase IIa trials were under way by February 2002 and it never reached approval [1][2].

A 24-week study of an oral AOD9604 candidate (534 enrolled, 502 randomized) failed to show significant benefit on the primary weight-loss endpoint, and the 2026 review that summarized it notes the conclusion rests on sponsor-reported phase 2 results rather than a peer-reviewed trial [2]. FDA lists AOD-9604 in its table of compounding nominations withdrawn by their nominators, and states that it has identified no, or only limited, safety information and that serious adverse events may be associated with it though causality is not clear [9].

What does diphenhydramine do?

Each Benadryl Allergy tablet contains 25 mg of diphenhydramine HCl. The label directs adults and children 12 and older to take 1 to 2 tablets every 4 to 6 hours, no more than 6 times in 24 hours [3]. The label says marked drowsiness may occur, says to avoid alcoholic drinks, notes that alcohol, sedatives and tranquilizers may increase drowsiness, and says to be careful when driving or operating machinery [3]. It also says to ask a doctor before use with glaucoma, trouble urinating due to an enlarged prostate, or a breathing problem such as emphysema or chronic bronchitis [3].

In an Iowa Driving Simulator crossover of 40 licensed drivers aged 25 to 44 with seasonal allergic rhinitis, diphenhydramine 50 mg produced the poorest overall driving performance of the four treatments tested, and self-reported drowsiness did not predict impairment [10].

Is diphenhydramine's CYP2D6 inhibition relevant to AOD-9604?

It is unknown. Diphenhydramine is a potent competitive CYP2D6 inhibitor in vitro, with an inhibitory constant of 2 micromol/L [7]. In 16 healthy men, steady-state diphenhydramine cut metoprolol's oral and nonrenal clearance about twofold and its alpha-hydroxylation clearance 2.5-fold in extensive metabolizers, but not in poor metabolizers [7]. That study tested metoprolol, not AOD-9604. Separately, researchers incubating AOD-9604 in serum and urine identified six potential metabolites, one of them more stable than the parent peptide [5]; that work did not involve diphenhydramine.

Who carries the most risk from the antihistamine?

Beers 2023 lists oral diphenhydramine among the first-generation antihistamines to avoid in adults 65 and older (moderate quality of evidence, strong recommendation): they are highly anticholinergic, clearance falls with age, tolerance develops when one is used as a hypnotic, and the risks include confusion, dry mouth and constipation [4][8]. The criteria also advise avoiding anticholinergics in dementia, in delirium or high delirium risk, and with a history of falls, while noting diphenhydramine may be appropriate for acute treatment of severe allergic reactions [8]. The prescription injection label adds that patients about 60 and older are more likely to have dizziness, sedation and hypotension [6].