How Long After Zyrtec Can You Take Benadryl?

In our latest question and answer, the pharmacist discusses how long after taking Zyrtec you can safely take Benadryl.


I’m having an allergic reaction from going through musty files from last night for 2:5 hours. My nose wouldnt stop running all day today. I took a dissolvable Zyrtec at 5:30 PM, can I now take a Benadryl tonight at 11:30 PM?

Asked by Patti On Jun 23, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Jun 24, 2018

Taking Zyrtec and Benadryl at the same time should only be done with caution due to the risk of increased adverse events. Both medications are antihistamines and taking them together greatly increases the risk of side effects such as:

  • Drowsiness
  • Dry mouth
  • Dizziness
  • Constipation
  • Dry eyes

Taking Benadryl After Zyrtec

Zyrtec (cetirizine) is a "second-generation" antihistamine, with a duration of action around 24 hours. To best avoid additive side effects, Benadryl should ideally be taken 24 hours after Zyrtec. However, taking Benadryl if you are having an allergic reaction is generally considered safe, even if you are already taking an antihistamine for your daily allergies. While you may be at an increased risk of certain side effects (such as drowsiness), they typically are mild in severity and the benefits of taking Benadryl to lessen the effects of an allergic reaction outweighs the risk of the mild side effects.

In addition, your doctor may recommend to take you take Benadryl for allergic symptoms that are severe, and don't seem to lessen after taking your Zyrtec. Benadryl tends to have better anti-inflammatory and anti-pruritic (i.e. anti-itch) effects than other antihistamines so use is often warranted. In general, they just shouldn't be taken together on a daily basis, only as needed intermittently.

About the Pharmacist

Dr. Adam DeRue Pharm.D

Dr. Adam DeRue is a Doctor Of Pharmacy and registered pharmacist.

About Benadryl (Diphenhydramine)

Benadryl (diphenhydramine) is a first generation, sedating antihistamine (H1-blocker) of the ethanolamine class. It is available in oral, topical, and parenteral product forms. Benadryl is well known to cause significant sedation in most individuals and is commonly used in night-time sleep aids. Benadryl also has potent "drying" effects in most patients, which is why is causes dry mouth, dry eyes and constipation. The onset of action of Benadryl is around 15-30 minutes after taking by mouth, with peak effects occurring 1 to 3 hours later. The duration of action is around 4 to 6 hours, with this being prolonged in the elderly and in those with liver disease.

About Zyrtec (Cetirizine)

Zyrtec (cetirizine) is a "second-generation" antihistamine, indicated for the treatment and prevention of perennial allergies, seasonal allergies and hives. At indicated doses (5mg and 10mg), it generally has a higher incidence of sedation than other second-generation antihistamines, such as Allegra (fexofenadine) and Claritin (loratadine). Nevertheless, it still causes less sedation than older, first-generation antihistamines such as Benadryl. Zyrtec is taken once daily, with an onset of action around one hour.

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