Why these two are so similar

Of all the comparisons on this site, this is the one where the two drugs have most in common. Both are first-generation H1 antihistamines, meaning both were developed in the era before medicinal chemists learned to keep antihistamines out of the brain.

Both cross the blood-brain barrier readily. Once inside, both block histamine signalling involved in maintaining wakefulness, which produces sedation. Both also have anticholinergic activity, so both can cause dry mouth, blurred close-up vision, constipation and difficulty starting urination.

They are different molecules and their labeling differs, but from the perspective of "what will this do to my alertness and my mouth", they belong to the same family and behave in family-like ways.

The difference that actually matters day to day

Availability. Hydroxyzine requires a prescription, which means a clinician has assessed the person, considered their other medicines and conditions, and specified a dose and pattern of use. Diphenhydramine is bought off a shelf with none of that.

This is not a claim that one is safer. It is a claim about what surrounds the decision. Over-the-counter status makes a medicine convenient and also makes it easy to take without anyone reviewing whether it is a good idea — which matters for a drug class with real anticholinergic and sedation risks, particularly in older adults.

The hidden-ingredient problem

Diphenhydramine appears in a remarkable number of products: most over-the-counter sleep aids, many nighttime pain relievers, combination allergy products, some motion-sickness products, and some anti-itch preparations. In several of these the front of the package gives no indication.

Someone taking a prescribed sedating antihistamine who also takes a "PM" pain reliever has taken two sedating antihistamines. That is the single most common way people accidentally stack this drug class, and it is entirely preventable by reading the active ingredients panel.

Where their labeled uses diverge

Hydroxyzine's labeling covers symptomatic relief of anxiety and tension, pruritus from allergic conditions, and sedation around procedures. Diphenhydramine products carry labeling covering allergy symptoms, itching, motion sickness, and — in specific products — use as a nighttime sleep aid.

That last point is a real difference worth noting. Certain over-the-counter diphenhydramine products are labeled as sleep aids; hydroxyzine has no such approved use, and when prescribed for sleep it is being used off-label.

This does not mean one is a better sleep option than the other. It means the regulatory status of that particular use differs, and a prescriber choosing between approaches is weighing far more than which product has which label.

Both warrant caution in older adults

First-generation antihistamines as a class appear on the American Geriatrics Society Beers Criteria list of medicines that often carry more risk than benefit in adults 65 and older. The reasons apply to both drugs: sedation, confusion, anticholinergic effects, and fall risk.

Because diphenhydramine is available over the counter, older adults frequently take it without any clinician being aware — often as a sleep aid, which is exactly the pattern the Beers Criteria flags. The National Institute on Aging notes that medicines causing dizziness or sedation contribute to falls, and falls in this age group frequently have serious consequences.

The practical takeaway for anyone over 65, or caring for someone over 65: over-the-counter sleep aids and nighttime products belong on the medicine list and in the conversation with a pharmacist, exactly like prescriptions do.

We will not tell you which to take

This page does not rank them, and that is deliberate rather than evasive.

The choice depends on the specific symptom, the person's other conditions and medicines, their age, what has been tried, and whether a clinician has assessed them. Those inputs are not available to a website, and a general recommendation would be wrong for a substantial share of readers.

What we can say is that the question people usually should be asking is not "which of these two" but "should I be taking a sedating antihistamine at all for this, and if so, is anything else I take already doing the same thing?" A pharmacist can answer that in a couple of minutes at no cost.