How long after taking Zyrtec can you take Benadryl?

Key takeaways

• Combining Benadryl and Zyrtec is generally not recommended due to the increased risk of side effects like drowsiness and rapid heart rate, which could lead to an antihistamine overdose in severe cases.
• If a doctor approves taking both antihistamines, it’s important to space out the doses, waiting 6 hours after Benadryl to take Zyrtec, or 24 hours after Zyrtec to take Benadryl, to minimize potential side effects.
• Benadryl and Zyrtec have different side effects. Benadryl (diphenhydramine) is a first-generation antihistamine that can cause drowsiness, while Zyrtec (cetirizine) is a second-generation antihistamine that typically doesn’t cause as much drowsiness.

Overview

Benadryl and Zyrtec are over-the-counter (OTC) antihistamine medications that may help relieve symptoms of allergies, such as:

• sneezing
• itchy, watery eyes
• runny nose
• itchy throat

Both Benadryl and Zyrtec are brand-name medications. Benadryl contains the active ingredient diphenhydramine, while Zyrtec contains the active ingredient cetirizine. An active ingredient is what makes a drug work.

Although both medications relieve similar symptoms, they differ in how quickly they work, how long they last, and their side effects. If you do not find relief with one medication, you may wonder whether you can mix them. The answer is — only in some instances.

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How long should you wait between taking Benadryl and Zyrtec?

Doctors typically do not recommend mixing oral antihistamines like diphenhydramine (Benadryl) and cetirizine (Zyrtec). This is because both antihistamines can cause side effects that may become more severe if you take the medications together. This could lead to antihistamine overdose. 

In some cases, your doctor may recommend taking both medications, but not at the same time or too close together. If they determine that it’s safe for you to take both drugs, they will likely want you to space them out to minimize the risk of potential side effects.

Some of the main differences between Benadryl and Zyrtec are how quickly they work and how long their effects last. Benadryl starts working within 15 to 30 minutes, while Zyrtec takes about 1 hour to start working.

Benadryl lasts 4 to 6 hours, so you may need to take it several times per day. It’s important not to 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 dose. Do not exceed more than 10 mg each day.

If your doctor determines that taking both medications may relieve your allergy symptoms, they may recommend taking Zyrtec 6 hours after taking Benadryl, which is how long Benadryl’s effects last.

If you take Zyrtec first, you should wait 24 hours before taking Benadryl. This can help prevent potential side effects from becoming too severe.

If you have allergies and use OTC drugs, it’s always a good idea to talk with your doctor. They can recommend which drug will be the most helpful for you and your allergies. They can tell you about possible interactions, contraindications (reasons the medications may not be suitable for you), and side effects.

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Why should you wait between doses?

Although Benadryl and Zyrtec work in slightly different ways to relieve allergy symptoms, they can lead to increased drowsiness, dizziness, and rapid heart rate in high doses. Taking both medications together can also lead to antihistamine overdose. The symptoms may include:

• confusion
• difficulty coordinating movements
• headache
• blurred vision
• vomiting
• fever
• stomach pain
• rapid heart rate
• severe drowsiness

Benadryl side effects

Diphenhydramine (Benadryl) can have side effects that include:

• drowsiness
• dizziness
• nausea
• vomiting
• loss of appetite
• constipation
• dry mouth, nose, and throat
• headache
• excitement, agitation, and nervousness

Zyrtec side effects

Cetirizine (Zyrtec) is a newer antihistamine associated with fewer side effects, which may include:

• drowsiness
• excessive tiredness
• dry mouth
• vomiting
• stomach pain
• diarrhea

If any of these side effects don’t go away, be sure to let your doctor know.

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. This is 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 or second-generation antihistamine. Although it blocks the effect of histamine the same as Benadryl, the Zyrtec drug molecule is unable to enter the CNS, so it doesn’t typically cause drowsiness.

Can you take Zyrtec and Benadryl 12 hours apart?

It is best not to mix Zyrtec and Benadryl or take them too close together. Always check with your doctor or healthcare professional if you want to take more than one of these medications to help relieve your allergy symptoms. Taking them both within a short space of time could lead to serious side effects.

Depending on your symptoms, overall health, and other medications you’re taking, your doctor may determine that it’s safe to take these drugs within the following timeframes:

• If you take Benadryl first, it may be safe to take Zyrtec 6 hours after you’ve taken Benadryl
• If you take Zyrtec first, it may be safe to take Benadryl 24 hours after you’ve taken Zyrtec

The effects of Zyrtec last a lot longer than the effects of Benadryl. This is why you need to wait longer after taking Zyrtec if you want to take Benadryl, too.

Recommended dosages of Benadryl and Zyrtec

Diphenhydramine (Benadryl) comes in 12.5-mg chewable tablets, 25-mg and 50-mg tablets, and a 12.5-mg/5 milliliter (mL) oral solution. The recommended dosage for Benadryl varies by age:

• Adults and children ages 12 years and older: Take 25- or 50-mg tablets every 4 to 6 hours. Do not exceed 300 mg per day.
• Children ages 6 to 11 years: Take 12.5 to 25 mg every 4 to 6 hours. Do not exceed 150 mg per day.
• Children younger than 6 years should not take Benadryl.

Cetirizine (Zyrtec) comes in 5- or 10-mg tablets and 5 mg/5 mL syrup or elixir. The recommended dosage for Zyrtec varies by age:

• Adults and children ages 12 years and older: Take one 5- or 10-mg tablet per day.
• Children ages 6 to 11 years: Take 1 teaspoon (5 mg) or 2 teaspoons (10 mg) of syrup once per day.
• Children ages 2 to 5 years: Take 1/2 teaspoon (2.5 mg) of syrup once per day.
• Children ages 6 to 23 months: Take 1/2 teaspoon (2.5 mg) of syrup once per day.

What are some other common allergy medications?

Other types of OTC and prescription antihistamines can also help relieve your symptoms. A medical professional may prescribe allergy medications if your symptoms are severe but may recommend OTC options if you have milder symptoms. Some OTC and prescription allergy medications include:

• OTC allergy medications:
  • loratadine (Claritin)
  • chlorpheniramine (Chlor-Trimeton)
  • pseudoephedrine (Sudafed)
  • fexofenadine (Allegra)
• Prescription allergy medications:
  • desloratadine (Clarinex)
  • montelukast sodium (Singulair)
  • mometasone suspension (nasal spray) (Nasonex)
  • prednisone (Rayos)
  • cromolyn (Gastrocrom)

Summary

Relieving allergy symptoms with OTC medications like Benadryl or Zyrtec is common. However, it’s best to avoid mixing them or taking them too close together. Doing so could lead to worsened side effects like increased drowsiness, dizziness, or a fast heart rate. It can also lead to an antihistamine overdose, which could cause serious complications.

If your doctor determines it’s safe for you to take both medications, they will typically suggest that you take them at different times. For instance, if you take Zyrtec first, they’ll likely tell you to wait at least 24 hours before taking Benadryl. And if you take Benadryl first, they’ll usually tell you to wait at least 6 hours before taking Zyrtec. 

If you have allergies, talk with a healthcare professional. They can help determine the best treatment for your needs.

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