Rexall Allergy Relief and Benadryl contain the same active ingredient at the same strength: diphenhydramine HCl 25 mg per tablet. They work identically in your body, treat the same symptoms, and follow the same dosing instructions. The only differences are the brand name on the box, the price, and some of the inactive filler ingredients used to form the tablet.
Why They’re Functionally Identical
Diphenhydramine is the drug doing all the work in both products. It’s a first-generation antihistamine that blocks the chemical your body releases during an allergic reaction, relieving symptoms like a runny nose, itchy or watery eyes, hives, and swelling. Because both tablets deliver exactly 25 mg of diphenhydramine hydrochloride, there is no pharmacological difference between taking one versus the other.
The FDA requires generic and store-brand medications to be bioequivalent to the brand-name version before they can be sold. Bioequivalence means the drug enters your bloodstream at the same rate and reaches the same concentration. Diphenhydramine products have been tested this way repeatedly, and generics consistently meet the standard. So while you’re paying less for the Rexall version, you’re not getting a weaker or slower-acting product.
Where They Differ: Inactive Ingredients
The active ingredient is identical, but the inactive ingredients (the fillers, coatings, and dyes that hold the tablet together) are not. Rexall Allergy Relief tablets contain corn starch, a red dye (D&C red #27 aluminum lake), calcium phosphate, magnesium stearate, microcrystalline cellulose, polyethylene glycol, polyvinyl alcohol, silicon dioxide, stearic acid, talc, and titanium dioxide. Benadryl’s tablet formulation uses a different combination of binders and coatings.
For most people, this distinction is irrelevant. But if you have a known sensitivity or allergy to a specific dye, filler, or binding agent, it’s worth comparing the two labels side by side. A reaction to one product’s inactive ingredients doesn’t mean you’ll react to the other.
Dosing and How Quickly It Works
The dosing is the same for both products. Adults and children 12 and older take one to two tablets every four to six hours as needed, with a maximum of six doses in 24 hours. Diphenhydramine reaches its peak level in your bloodstream about 1.5 hours after you take it, though most people start feeling the effects sooner than that. Its oral bioavailability is around 72%, meaning roughly three-quarters of the dose actually makes it into your system.
Side Effects Apply to Both
Because the active drug is the same, both products carry identical side effects. Drowsiness is the big one. Diphenhydramine is a first-generation antihistamine, which means it crosses into the brain easily and causes sedation. This is why it’s also the active ingredient in many over-the-counter sleep aids. Other common effects include dizziness, dry mouth, impaired coordination, blurred vision, and constipation.
The sedation isn’t just inconvenient. Diphenhydramine can impair your ability to drive or operate machinery, and it increases the risk of falls in older adults due to a combination of dizziness, sedation, and drops in blood pressure. For this reason, many geriatric guidelines flag it as a high-risk medication for people over 65.
People with asthma, high blood pressure, cardiovascular disease, overactive thyroid, or increased eye pressure should use diphenhydramine cautiously regardless of which brand they buy. In children, the drug can sometimes cause the opposite of drowsiness: confusion, agitation, and restlessness, a reaction known as paradoxical stimulation.
When a Newer Antihistamine Might Be Better
If you’re taking allergy medication daily for seasonal or environmental allergies, diphenhydramine (whether Rexall or Benadryl) usually isn’t the best choice. Its effects only last four to six hours, and the drowsiness can interfere with your day. Second-generation antihistamines like cetirizine (Zyrtec), loratadine (Claritin), or fexofenadine (Allegra) last 24 hours and cause far less sedation. Diphenhydramine works well for acute situations like a sudden allergic reaction, hives, or occasional nighttime allergy symptoms where the sedation is less of a problem.
The Bottom Line on Price vs. Brand
Rexall Allergy Relief is a store-brand generic. You’re getting the same 25 mg of diphenhydramine that’s in every Benadryl tablet, manufactured to the same FDA purity and bioequivalence standards. The price difference is purely a function of brand recognition and marketing costs. Choosing one over the other is a financial decision, not a medical one.