Benadryl and Zyrtec are over-the-counter (OTC) allergy medications that may help relieve sneezing, itchy and watery eyes, runny nose, and coughing.
They’re brand names for medications containing the active ingredients diphenhydramine and cetirizine.
Both are oral antihistamines designed to provide relief from similar symptoms. However, they differ in how long they last and the side effects they produce, and sometimes, you may consider taking both antihistamines to relieve symptoms.
If you can’t find relief with one medication, you may wonder whether you can mix them, but the answer is — only in some instances.
How long to wait before taking Benadryl after taking Zyrtec
Healthcare professionals typically don’t recommend mixing oral antihistamines like diphenhydramine (Benadryl) and cetirizine (Zyrtec). They don’t advise this because both antihistamines can cause side effects that can become stronger if you take them together and could lead to antihistamine overdose.
However, in some cases, they may recommend taking both medications but with a specific amount of time between types and doses. One of the main differences between Benadryl and Zyrtec is how long their effects last.
Benadryl lasts 4–6 hours, so you might have to take it several times per day, but do not exceed more than six 25-milligram (mg) tablets in a day.
Zyrtec lasts up to 24 hours, so you may only need to take it once per day, depending on the dosage. Do not exceed more than 10 mg each day.
Your healthcare professional may think taking both medications can relieve your allergy symptoms, so they may recommend taking Zyrtec 6 hours after taking Benadryl, which is how long Benadryl’s effects last.
If you take Zyrtec first, wait 24 hours before taking Benadryl. This can prevent potential adverse effects from worsening.
How does each drug work?
Diphenhydramine (Benadryl) and cetirizine (Zyrtec) are antihistamines that belong to different generations.
Diphenhydramine (Benadryl) is a first-generation antihistamine that works directly in your central nervous system (CNS) to block histamine — a chemical your immune system releases when it encounters allergens like pollen, dust, and pet dander. First-generation antihistamines tend to cause drowsiness because they work on the CNS.
Cetirizine (Zyrtec) is a newer antihistamine belonging to the second generation. It doesn’t block the effect of histamine as diphenhydramine (Benadryl) does, as it avoids the CNS. So, it doesn’t typically cause drowsiness.
Recommended dosage
Diphenhydramine (Benadryl) comes in 25-mg tablets. The recommended dosage for Benadryl varies by age:
- Adults and children 12 years and older: Take one or two tablets every 4–6 hours. Do not exceed six tablets per day.
- Children 6–11 years old: Take one tablet every 4–6 hours.
- Children younger than 6 years old should not take Benadryl.
Cetirizine (Zyrtec) comes in 5-mg or 10-mg tablets and 5-mg syrup or elixir. The recommended dosage for Zyrtec also varies by age:
- Adults and children 12 years and older: Take one 5-mg or 10-mg tablet per day.
- Children 6–11 years old: Take 1 teaspoon (5 mg) or 2 teaspoons (10 mg) of syrup form once per day.
- Children 2–5 years old: Take 1/2 teaspoon (2.5 mg) of syrup form once per day.
- Children 6–23 months old: Take 1/2 teaspoon (2.5 mg) of syrup form once per day.
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