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.
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.
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.
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.
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.
SummaryTheraflu 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.