How Long Does Benadryl (Diphenhydramine) Last

Our pharmacist answers the latest question regarding how long the antihistamine Benadryl (Diphenhydramine) lasts.


Hello, knowing that Benadryl can cause drowsiness, I'm wondering for how long. Is the drowsiness 24 hours until the next tablet is taken? For example, if someone takes a benadryl at10pm will the medication still be in effect for 24 hrs and will the drowsiness last that long?or is would it only be through the night when it's irrelevant because the person is sleeping? Thanks.

Asked by Cready On Sep 29, 2017

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Sep 29, 2017
Pink Benadryl Tablets


Benadryl (Diphenhydramine) does not typically last 24 hours. That duration of action is associated with the 'second-generation' antihistamines, like Claritin and Allegra.

Each dose of Benadryl will last approximately 4 to 6 hours, but this can vary by individual.

For example, it typically lasts longer in the elderly, which is why it sometimes causes a 'hangover' effect in older individuals if taken as a sleep aid.

Benadryl Duration And Onset

As stated, Benadryl is a fairly short-acting antihistamine, lasting around 4 to 6 hours per dose.

The onset of action is fairly rapid, occurring in 15-30 minutes after taking a dose by mouth, with peak concentrations in the blood occurring after around 2 hours. Therefore, if you are planning on using as a sleep aid, be sure to take it around 15-30 minutes before bedtime.[1][2]

The maximum sedative effect occurs between 1—3 hours after taking the drug.

Benadryl is classified as a "first-generation" antihistamine and has similar characteristics (e.g. short duration of action, sedating) to the other drugs in that class, which include:

  • Tavist (clemastine)
  • Doxylamine
  • Brompheniramine
  • Chlor-Trimeton (Chlorpheniramine)
Section Summary
Benadryl (diphenhydramine) is a short acting, sedating antihistamine. It will begin to work in 15-30 minutes and each dose lasts on average four to six hours.

Hangover Effect

Due to the short duration of action, it is relatively uncommon for someone to have a "hangover" effect when used for sleep. However, the drug can last longer in certain individuals, including:[3]

  • The elderly
  • Those with liver dysfunction
  • Those taking medications that prolong the metabolism of Benadryl

In general, to avoid the possibility of a 'hangover effect', it is recommended to try and get a good 7-8 hours of sleep if using the drug before bed.


Benadryl can be taken with or without food.

The typical dose of Benadryl is:

  • 25-50 mg every 4-6 hours for allergy symptoms.
  • 25-50 mg at bedtime for insomnia.


For most individuals, Benadryl (diphenhydramine) lasts four to six hours per dose.

  1. ^ DrugBank Diphenhydramine Monograph. DrugBank
  2. ^ Elsevier ClinicalKey: Diphenhydramine Monograph. ClinicalKey
  3. ^ Pharmacological Treatment of Insomnia. PubMed

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Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Office: 716-389-3076

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