Samuel asked
I often take Benadryl and Zyrtec. Can you take them together?
At a glance
- There are some situations where your doctor may recommend the Benadryl and Zyrtec together. You should only do so under their recommendation.
Answer
Overview
Generally, you wouldn't want to take Zyrtec (cetirizine) and Benadryl (diphenhydramine) at the same time.
They are both antihistamines and combining them is considered a therapeutic duplication.
Having said that, there certainly are situations where both may be recommended together.
The Interaction
Benadryl is a sedating antihistamine, also known as a 'H1-blocker'.
Taking Benadryl with Zyrtec will have effects. This increases the risk of certain side effects, including:
- Dry mouth
- Drowsiness
- Dry eyes
- Sedation
- Constipation
Zyrtec (cetirizine), although a 'second-generation antihistamine' (like Claritin and Allegra) is not considered to be 'non-drowsy'.
Clinical trials for the drug show about 10-15% of individuals will experience sedation or a general feeling of sleepiness.
The prescribing information for Zyrtec recommends being cautious when it comes to activities such as driving a car:
"...in clinical trials, the occurrence of CNS effects [with Zyrtec] has been observed in some individual patients and due caution should be exercised when driving a car or operating potentially dangerous machinery."
Aside from the fact that both Benadryl and Zyrtec can cause sedation, they have the same mechanism of action.
They both work by binding to histamine-1 receptors in the body, preventing histamine from binding to them.
This helps to prevent or reduce the allergy related symptoms (e.g. runny nose, stuffy nose, post-nasal drip etc...).
Taking both will not have a significant additive effect when it comes to helping with your allergy symptoms.
- For example, if you take a dose of Zyrtec, that drug will bind to the histamine receptors. If you take Benadryl shortly after your Zyrtec dose, the histamine receptors are already bound to Zyrtec, greatly reducing any potential additive effect.
Overall, due to the same mechanism of action, and similar side effect profile, concurrent use of sedating antihistamines (H1-blockers) is not recommended.
Final Words
- Zyrtec (cetirizine) and Benadryl (diphenhydramine) are both antihistamines are generally aren't recommended to be used together.
- They have the same mechanism of action and a similar side effect profile.
- There may be situations where both can be used together intermittently, such as for an allergic reaction or the treatment of insomnia.
References
- Zyrtec Product Information. Medicines.org
- Antihistamines, drowsiness, and psychomotor impairment: central nervous system effect of cetirizine. PubMed
- Comparative anticholinergic activities of 10 histamine H1 receptor antagonists in two functional models. PubMed
- Pharmacology of Antihistamines. PubMed
- Anticholinergic potency of diphenhydramine (Benadryl) measured against bethanechol in the gastric fistula dog. PubMed
