Reactine vs Benadryl: which allergy medication works best for me?

Key takeaways

• For most people, Reactine (cetirizine) is the better everyday allergy medication. It causes less drowsiness, lasts about 24 hours and has very little effect on memory and thinking. Benadryl (diphenhydramine) works fast but makes most people sleepy.
• Older allergy drugs are not meant for daily use. Benadryl, Gravol and similar "first-generation" antihistamines block a brain chemical needed for memory. Using them regularly for years, for example as a sleep aid, is linked to a higher risk of dementia.
• Your genes can change how these drugs work for you. Most older antihistamines are broken down by an enzyme called CYP2D6. People who are poor metabolizers may feel stronger and longer side effects; ultrarapid metabolizers may find the drugs wear off quickly.
• Check before combining with other medications. Benadryl can raise the levels of some antidepressants and heart drugs such as metoprolol. Ask a pharmacist before buying an over-the-counter allergy product.
• Antihistamines do not replace epinephrine. If you have severe allergies, carry your EpiPen at all times.

Reactine or Benadryl: which is better? For regular allergy control, Reactine (cetirizine) and other second-generation antihistamines such as Claritin (loratadine) and Allegra (fexofenadine) are the better choice for most adults. Benadryl (diphenhydramine) is an older, first-generation antihistamine that acts quickly but causes drowsiness, dry mouth and confusion, interacts with many medications through the CYP2D6 enzyme, and should be avoided for regular use, especially by adults 65 and older.[1][2]

Antihistamines are key medications for many people living with food or seasonal allergies. Many people also use them for nausea, motion sickness or as sleep aids. When used occasionally, these over-the-counter medications are generally safe. But regular, long-term use, particularly of the older first-generation antihistamines, can cause serious health problems that you may not notice until it is too late.

Severe food, insect-sting and drug allergies can be life-threatening. If you are at risk, carrying your epinephrine auto-injector (EpiPen) and your allergy medications at all times is essential. Antihistamines can ease milder symptoms, but they do not replace epinephrine in a severe reaction (anaphylaxis).

What are first-generation and second-generation antihistamines?

Antihistamines fall into two groups. First-generation antihistamines are older drugs that easily cross into the brain, so they cause drowsiness and have strong anticholinergic effects. Anticholinergic means that a drug blocks acetylcholine, a chemical messenger that nerve cells use for memory and attention and to control automatic body functions such as saliva, digestion, bladder emptying and heart rate. This is what causes typical side effects such as dry mouth, constipation, blurred vision, difficulty passing urine and confusion. Second-generation antihistamines are newer drugs that mostly stay out of the brain, so they cause much less drowsiness. The table below lists common drugs, their brand names and the liver enzymes that break them down. These enzymes matter because the genes that encode them differ from person to person.

Drug (generic name)	Common brand names (Canada / US)	Main liver enzyme(s) involved	Good to know
First-generation antihistamines (older, sedating, strongly anticholinergic)
Diphenhydramine	Benadryl; Nytol, Sleep-Eze and many "PM" products	CYP2D6 (main); also blocks CYP2D6	Strongly sedating and anticholinergic
Dimenhydrinate	Gravol (Canada); Dramamine (US)	CYP2D6 (as diphenhydramine)	Strongly sedating and anticholinergic
Chlorpheniramine	Chlor-Tripolon (Canada); Chlor-Trimeton (US); many cold products	CYP2D6 (main)	Often used in combination cold remedies
Brompheniramine	Found in combination cold and allergy products	CYP2D6	Check labels of multi-symptom products
Doxylamine	Diclectin; Unisom; "night-time" cold products	CYP2D6, CYP1A2, CYP2C9	Used for sleep and pregnancy nausea
Promethazine	Phenergan (prescription)	CYP2D6 (main)	Used for allergy, nausea and sedation
Meclizine	Bonamine (Canada); Antivert, Bonine (US)	CYP2D6	Used for vertigo and motion sickness
Hydroxyzine	Atarax (prescription)	CYP3A4/5 (converted to cetirizine)	Not affected by the CYP2D6 gene
Second-generation antihistamines (newer, less sedating)
Cetirizine	Reactine (Canada); Zyrtec (US)	Minimal liver metabolism; cleared unchanged by the kidneys	Can still cause drowsiness in some people
Levocetirizine	Xyzal	Minimal liver metabolism; kidney clearance	Active form of cetirizine
Fexofenadine	Allegra	Minimal liver metabolism; handled by transporter proteins (P-gp, OATP)	Red orange and grapefruit juice can lower absorption
Loratadine	Claritin	CYP3A4 and CYP2D6 (converted to desloratadine)	Less sedating
Desloratadine	Aerius (Canada); Clarinex (US)	UGT2B10 and CYP2C8	Active metabolite of loratadine

Enzyme information is based on drug labelling and published pharmacokinetic studies; the evidence is strongest for diphenhydramine, chlorpheniramine and promethazine.[3]

How do your CYP2D6 genes affect Benadryl and Gravol?

Most first-generation antihistamines (with the exception of hydroxyzine) depend mainly on the liver enzyme CYP2D6. The gene encoding this enzyme is highly variable across individuals. Some people have very low or completely absent CYP2D6 function (poor metabolizers). Others carry extra functional copies of the gene (ultrarapid metabolizers) and break drugs down unusually fast.[3]

For poor metabolizers, CYP2D6-dependent drugs such as Benadryl and Gravol may feel stronger and last longer because they stay in the bloodstream longer. The flip side is more drowsiness and a higher chance of side effects. Ultrarapid metabolizers eliminate these medications faster, so the drugs may wear off quickly or seem ineffective.

If Benadryl and Gravol seem unusually strong for you

If you have noticed that Benadryl and Gravol work better for you than for most people, you may be an intermediate or poor metabolizer for CYP2D6. It is not proof: other medications that block CYP2D6, such as paroxetine, fluoxetine and bupropion, can make a normal metabolizer act like a poor metabolizer. If this does apply to you, it may also mean that Tylenol 3 (codeine) and tramadol are less effective painkillers for you, and that some antidepressants are more likely to cause side effects.

If Reactine and Allegra work better for you than Benadryl

If you find that Reactine and Allegra, which do not depend on CYP2D6, work better for you than Benadryl or Chlor-Tripolon, you may be an ultrarapid metabolizer, and a pharmacogenetic test is worth considering. Ultrarapid metabolizers can convert codeine into morphine too quickly, which can lead to dangerous side effects, so knowing your status can help you avoid serious medication-induced complications.

A pharmacogenetic (PGx) test such as Pillcheck reports your CYP2D6 metabolizer status, along with other genes that affect how you respond to medications. Pillcheck reports on 24 genes, increasing coverage with new guidelines and drug labels, so one test can inform many medications you may need in the future.

Which medications interact with antihistamines?

Antidepressants

If you are taking certain antidepressants (including fluoxetine, paroxetine or amitriptyline), consult a pharmacist before buying an over-the-counter allergy drug.

• Fluoxetine (Prozac) and paroxetine (Paxil) strongly block CYP2D6, which can raise levels of first-generation antihistamines and increase drowsiness, confusion and dry mouth.
• Amitriptyline is itself strongly anticholinergic, so the effects add up.
• Venlafaxine (Effexor): in a study of healthy volunteers, diphenhydramine cut venlafaxine clearance by more than half in normal metabolizers by blocking CYP2D6.[5]
• Cold products with dextromethorphan ("DM") can add a risk of serotonin syndrome, a serious complication, when combined with some antidepressants.

Heart medications: metoprolol

Metoprolol, a beta-blocker used for high blood pressure, angina, heart failure, abnormal heart rhythms and sometimes anxiety, is also broken down by CYP2D6. In a clinical study of 16 healthy men, diphenhydramine roughly halved metoprolol clearance in people with normal CYP2D6 function, making its effects on heart rate and blood pressure stronger and longer.[4] In practice, this can cause a slow heartbeat, low blood pressure (hypotension), dizziness and fainting.

Is it safe to take antihistamines every day?

Second-generation antihistamines such as Reactine, Claritin and Allegra are generally preferred for long-term allergy control. First-generation antihistamines are a different story. While it might be tempting to keep using Benadryl or Gravol for vertigo, nausea or sleep, you should think twice about taking them regularly.

Antihistamines and dementia risk

Frequent use of strong anticholinergic drugs, including first-generation antihistamines, is associated with a higher risk of dementia, especially in older people. In a study that followed 3,434 adults aged 65 and older for an average of 7.3 years, those with the highest cumulative use (more than three years of a standard daily dose over 10 years) had about a 50% higher risk of dementia than non-users. This type of study shows an association, not proof of cause, but the signal was consistent and worrisome.[2]

Why do antihistamines affect memory?

Acetylcholine is a chemical messenger that nerve cells use to talk to each other. In the brain, it supports memory, learning and attention. In the rest of the body, it helps control saliva, bowel movements, bladder emptying and heart rate. Anticholinergic drugs block acetylcholine. First-generation antihistamines cross easily into the brain, which is why they cause drowsiness, brain fog and confusion, and why long-term use is a concern for memory. Other medications have a similar effect, such as the antidepressant amitriptyline and bladder drugs like oxybutynin.

For this reason, the 2023 American Geriatrics Society Beers Criteria advise adults 65 and older to avoid first-generation antihistamines, and pharmacists usually recommend that older adults stop regular use of anticholinergic medications.[1] Never stop a prescribed medication without talking to your doctor or pharmacist first. For safer sleep options, see Precision medicine and sleep medications.

Other side effects of antihistamines

• Daytime drowsiness and a "hangover" feeling, which can impair driving and increase falls and fractures in older adults.
• Confusion, memory problems and, in older people, delirium.
• Dry mouth (which also promotes tooth decay), constipation and blurred vision.
• Difficulty passing urine, especially for men with an enlarged prostate.
• Worsening of narrow-angle glaucoma.
• A fast or irregular heartbeat; some antihistamines (for example, hydroxyzine, and diphenhydramine in overdose) can prolong the QT interval of the heart rhythm.
• Loss of effect: as a sleep aid, diphenhydramine tends to stop working within days, and sleep quality can suffer.
• Paradoxical agitation or hyperactivity in some children, and dangerous effects in overdose.
• Severe itching after stopping long-term daily use of cetirizine (Reactine, Zyrtec) or levocetirizine (Xyzal). This is rare, but in 2025 the US FDA added a warning, so talk to your pharmacist if you plan to take them for more than a few months.[6]

How can I better manage my allergies?

The main goal is to eliminate or reduce your exposure to allergens.

• Find your triggers. A skin prick test with an allergist can help determine which foods or other triggers you should avoid. Blood tests for specific IgE can also be useful, but they should be interpreted by an allergist together with your history.
• Skip "food sensitivity" (IgG) tests. They are broadly marketed, but they have low accuracy (poor sensitivity and specificity) and are not recommended by allergy societies.
• Ask a pharmacist about pill ingredients. Drug-related allergies can be harder to solve because you may react to an inactive ingredient (excipient) of the pill, such as lactose, talc or food dyes. These reactions may occur when switching between different brands of the same medication. A pharmacist can help you find brands or generics without the offending excipient.

Summary

• Allergy medications are generally safe when used occasionally, but first-generation antihistamines such as Benadryl and Gravol can be harmful if taken regularly for a long time.
• For everyday allergy control, second-generation antihistamines such as Reactine, Claritin and Allegra are the better choice for most people.
• Your CYP2D6 genes may explain why some allergy medications work better for you, and whether they are safe with your other medications.
• If you have severe allergies, see an allergist to find the cause and avoid it.
• If you take several medications, ask a pharmacist to help you choose the allergy medication best suited to you.

Frequently asked questions

Which is better, Reactine or Benadryl?

For most people, Reactine (cetirizine) is the better everyday choice. It is less sedating, lasts about 24 hours and has minimal anticholinergic effects. Benadryl (diphenhydramine) acts faster and is used for acute reactions, but its drowsiness, anticholinergic effects and CYP2D6 interactions make it a poor choice for regular use.

What does anticholinergic mean?

An anticholinergic drug blocks acetylcholine, a chemical messenger the body uses for memory, attention, saliva, digestion, bladder control and heart rate. First-generation antihistamines such as Benadryl and Gravol are strongly anticholinergic, which explains their drowsiness, dry mouth, constipation and confusion. Other strongly anticholinergic drugs include amitriptyline and the bladder medication oxybutynin.

Is it safe to take Benadryl or Gravol every night to sleep?

It is not recommended. Tolerance to the sleep effect develops quickly, next-day grogginess is common, and regular use of anticholinergic drugs is linked to a higher dementia risk, especially in older adults. Ask your pharmacist about safer ways to improve sleep.

Can I take Reactine every day?

Second-generation antihistamines are generally preferred for long-term allergy control. Use the lowest effective dose, and discuss long-term use with your pharmacist. After months or years of daily use, stopping cetirizine can rarely cause severe itching, so do not stop abruptly without advice.

Is Gravol the same as Benadryl?

Not exactly, but close. Gravol's active ingredient, dimenhydrinate, is a salt of diphenhydramine, so it has similar sedation, anticholinergic effects and CYP2D6 interactions.

What is the safest antihistamine for older adults?

Second-generation antihistamines such as loratadine (Claritin) and fexofenadine (Allegra) are generally preferred, because adults 65 and older should avoid first-generation drugs. Cetirizine and levocetirizine are cleared by the kidneys, so they are not recommended for people with reduced kidney function. Check with a pharmacist.

Can I take antihistamines with antidepressants or heart medications?

Always check with your pharmacist or doctor if you are taking allergy medication. Drugs such as fluoxetine, paroxetine, amitriptyline, venlafaxine and metoprolol can interact with first-generation antihistamines. Ask a pharmacist to review your full medication list before you buy an over-the-counter product.

Can a pharmacogenetic test tell me which antihistamine is best?

A test of your CYP2D6 gene can show whether you are likely to clear first-generation antihistamines slowly or quickly, and the same result informs other medications such as codeine, tramadol, many antidepressants and some beta-blockers. There is no formal pharmacogenetic guideline for antihistamines yet, so results are best used together with a pharmacist's review of your medications and symptoms.

Does Pillcheck test CYP2D6?

Yes. Pillcheck reports your CYP2D6 metabolizer status (poor, intermediate, normal or ultrarapid metabolizer) as part of its 24-gene panel, along with how it may affect the medications you take.

This article is for education only and does not replace advice from your pharmacist or physician.

References

1. 2023 American Geriatrics Society Beers Criteria Update Expert Panel. American Geriatrics Society 2023 updated AGS Beers Criteria for potentially inappropriate medication use in older adults. J Am Geriatr Soc. 2023;71(7):2052-2081. doi:10.1111/jgs.18372
2. Gray SL, Anderson ML, Dublin S, et al. Cumulative use of strong anticholinergics and incident dementia: a prospective cohort study. JAMA Intern Med. 2015;175(3):401-407. doi:10.1001/jamainternmed.2014.7663
3. Sharma A, Hamelin BA. Classic histamine H1 receptor antagonists: a critical review of their metabolic and pharmacokinetic fate from a bird's eye view. Curr Drug Metab. 2003;4(2):105-129. doi:10.2174/1389200033489523
4. Hamelin BA, Bouayad A, Méthot J, et al. Significant interaction between the nonprescription antihistamine diphenhydramine and the CYP2D6 substrate metoprolol in healthy men with high or low CYP2D6 activity. Clin Pharmacol Ther. 2000;67(5):466-477. doi:10.1067/mcp.2000.106464
5. Lessard E, Yessine MA, Hamelin BA, et al. Diphenhydramine alters the disposition of venlafaxine through inhibition of CYP2D6 activity in humans. J Clin Psychopharmacol. 2001;21(2):175-184. doi:10.1097/00004714-200104000-00009
6. US Food and Drug Administration. FDA requires warning about rare but severe itching after stopping long-term use of oral allergy medicines cetirizine or levocetirizine (Zyrtec, Xyzal, and others). Drug Safety Communication, 16 May 2025. FDA.gov