Taking Zyrtec With Theraflu Nighttime Products

In our latest question and answer, the pharmacist discusses taking Zyrtec with Theraflu Nighttime products.


I took 1 pill of 10mg Zyrtec. Would I be able to take Theraflu Nighttime for severe cold?

Asked by Juan On Apr 19, 2019

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Apr 19, 2019
Pink Benadryl Pills


All of the Theraflu Nighttime products contain diphenhydramine (which is also the active ingredient in Benadryl).

So, when looking into a potential interaction between Zyrtec (cetirizine) and Theraflu Nighttime products, we are looking at two specific drugs:

All of the other ingredients that are contained in the Theraflu family of products (e.g. acetaminophen, dextromethorphan) are safe with Zyrtec (cetirizine).

We have written about the potential interaction between Zyrtec (cetirizine) and Benadryl (diphenhydramine) before, but I will go over the important points below.

What Is Diphenhydramine?

Diphenhydramine is a first-generation antihistamine and is included in several over the counter nighttime cold products due to its sedative properties. For example, it is the active ingredients in Vick's ZzzQuil. In Theraflu Nighttime, it is also the sedative ingredient.

First-generation antihistamines, like diphenhydramine, aren't generally used consistently for allergy prevention. They are relatively short-acting (around 4 to 6 hours) per dose and have potent sedative effects. Additionally, they can also cause dry mouth, dry eyes, and constipation.[1]

Diphenhydramine is most often used for the early treatment of allergic reactions, like contact dermatitis (e.g. to poison ivy). They are also used for the short-term treatment of insomnia.[2]

What Is Zyrtec?

Zyrtec is a 'second-generation' antihistamine, which are noted for their long duration of action (~24 hours per dose) and lack of sedative side effects.[3]

While Zyrtec does cause less sedation than diphenhydramine, it still has sedative properties and published data suggests that it causes drowsiness in around 10% of individuals.[4]

The characteristics of second-generation antihistamines (long duration of action, less drowsiness) are in contrast to first-generation antihistamines, which makes them more often utilized for daily use for allergy prevention.

Taking Cetirizine With Diphenhydramine

As a general rule of thumb, it isn't recommended to take multiple drugs from the same class of medication due to the increased risk of side effects and lack of additional benefit.

This is likely the case for taking Zyrtec with diphenhydramine (which is contained in Theraflu Nighttime products) since they are both antihistamines.

Nevertheless, intermittent use of both is, for the most part, likely safe.

Regardless of whether or not you take a daily antihistamine for allergies, diphenhydramine may be recommended by your doctor for use as an occasional sleep aid. It just shouldn't be used every day. The benefit of getting a good night sleep can often outweigh the risks of mild side effects that occur.

You may notice some additive side effects, like drowsiness (which shouldn't be an issue at bedtime) and dry eyes, but they likely won't be significant.

So overall, once in a while use of both generally isn't a problem, but if you use both on a consistent basis, that is when you may see the additive adverse reactions.


Theraflu Nighttime products contain diphenhydramine, a sedating first-generation antihistamine. Since Zyrtec is also an antihistamine, taking both can increase the risk of side effects like excess sedation and dry eyes. Intermittent use of diphenhydramine however, even if you are taking Zyrtec daily for allergy prevention, may be safe and recommended by your doctor for the short-term treatment of insomnia.

  1. ^ Elsevier ClinicalKey: Diphenhydramine Monograph. ClinicalKey
  2. ^ Diphenhydramine versus nonsedating antihistamines for acute allergic reactions: a literature review. PubMed
  3. ^ Pharmacology of Antihistamines. PubMed
  4. ^ Review of cetirizine hydrochloride for the treatment of allergic disorders. 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 PharmacistAnswers.com. 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! Brian.Staiger@PharmacistAnswers.com Office: 716-389-3076

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