Zyrtec (cetirizine) has very little anticholinergic activity. In lab studies, cetirizine showed no effect on muscarinic receptors even at concentrations 800 times higher than what’s needed to block histamine. It is classified as a second-generation antihistamine, a group specifically designed to avoid the anticholinergic side effects common with older allergy medications like Benadryl.
That said, cetirizine isn’t completely free of anticholinergic burden. It carries a score of 2 out of 3 on the Anticholinergic Burden Scale, which places it in a gray zone worth understanding, especially if you take other medications or are older.
How Cetirizine Compares to Anticholinergic Antihistamines
First-generation antihistamines like diphenhydramine (Benadryl), hydroxyzine, and doxylamine are strongly anticholinergic. They block acetylcholine receptors throughout the body, causing dry mouth, blurred vision, constipation, urinary retention, and confusion. These drugs are highly fat-soluble, which means they easily cross into the brain and interfere with signaling there too.
Cetirizine works differently. It targets histamine H1 receptors with high precision, blocking them at a concentration of about 0.12 micromoles. When researchers tested cetirizine against muscarinic (acetylcholine) receptors at concentrations up to 100 micromoles, it had zero measurable effect. For comparison, first-generation antihistamines block both receptor types at similar concentrations, which is why they cause so many non-allergy side effects.
Why Second-Generation Drugs Stay Out of the Brain
The key difference between older and newer antihistamines is how well they penetrate the brain. First-generation drugs dissolve easily in fat and pass freely through the blood-brain barrier, where they block histamine and acetylcholine receptors in areas that control alertness, memory, and coordination.
Cetirizine stays mostly outside the brain because of a protein called P-glycoprotein, an efflux pump embedded in the walls of blood vessels that supply the brain. This pump actively pushes cetirizine back out of the central nervous system. In a human study, when researchers gave cetirizine alongside a drug that blocks P-glycoprotein, participants became noticeably less alert and had slower reaction times. This confirmed that P-glycoprotein is actively keeping cetirizine out of the brain under normal circumstances, and that cetirizine’s favorable side effect profile depends partly on this transport mechanism.
The Anticholinergic Burden Score
Despite its minimal direct activity at muscarinic receptors, cetirizine is assigned a score of 2 on the Anticholinergic Burden (ACB) scale. This three-point scale rates drugs from 1 (possible anticholinergic effects based on lab data) to 3 (definite, clinically significant anticholinergic effects). A score of 2 means there is some clinical evidence of mild anticholinergic effects, even if the drug’s primary action is elsewhere.
This score matters most when you’re taking multiple medications. Anticholinergic burden is cumulative. If you take cetirizine (score of 2) alongside another medication with a score of 1 or 2, the combined burden can start producing noticeable effects like dry mouth, mild confusion, or constipation. In a hospital survey, only 20% of healthcare staff correctly identified cetirizine’s ACB score, and 18% believed it had no anticholinergic burden at all. This suggests the mild anticholinergic potential of cetirizine is widely underestimated.
What the 2023 Beers Criteria Say
The American Geriatrics Society maintains a list of medications considered potentially inappropriate for older adults, known as the Beers Criteria. The 2023 update specifically flags first-generation antihistamines (diphenhydramine, hydroxyzine, promethazine, doxylamine, and others) as “highly anticholinergic” and potentially inappropriate. Cetirizine does not appear on this list, nor is it included in the table of drugs with strong anticholinergic properties.
This is a meaningful distinction. It means that, based on current evidence, geriatrics experts do not consider cetirizine’s anticholinergic effects significant enough to warrant the same caution applied to older antihistamines.
Effects on Memory and Thinking
Because cetirizine does reach the brain in small amounts, researchers have studied whether it affects cognitive performance. In a controlled crossover study, single doses of 10 mg and 20 mg cetirizine were compared to placebo. Both doses slightly slowed the speed at which participants could scan through items held in working memory. The 10 mg dose also mildly impaired the ability to track a moving target on screen.
However, neither dose affected the ability to learn or recall new words, a measure of episodic memory. The researchers described the overall cognitive effects as “marginal,” concluding that cetirizine’s presence in the brain was not sufficient to produce clear, measurable impairment. These were also single-dose studies in healthy volunteers, not long-term trials, so they reflect what might happen on a given day rather than cumulative risk over months or years.
Practical Takeaways
Cetirizine is not a clinically significant anticholinergic drug. It does not bind to muscarinic receptors at therapeutic doses, it is actively pumped out of the brain, and it is not flagged on the Beers Criteria alongside truly anticholinergic antihistamines. For most people, it is a safe choice that avoids the dry mouth, blurred vision, and cognitive fog associated with first-generation alternatives.
Where it deserves more thought is in people already taking other medications with anticholinergic properties, or in older adults managing a high overall medication burden. The ACB score of 2 is not alarming on its own, but it adds up. If you’re concerned about cumulative anticholinergic load, alternatives like fexofenadine (Allegra) and loratadine (Claritin) showed no muscarinic receptor activity at all in the same studies that tested cetirizine, and they may be worth considering.