Zyrtec (cetirizine) is one of the most widely used over-the-counter allergy medications, but it causes more side effects than many people expect from a “non-drowsy” antihistamine. Drowsiness is by far the most common issue, affecting 11 to 14 percent of users at the standard dose. Beyond that, dry mouth, fatigue, and digestive complaints round out the list of effects most people notice. There’s also a lesser-known withdrawal effect, intense itching after stopping the drug, that the FDA recently required to be added to the label.
Drowsiness Is More Common Than You’d Think
Zyrtec belongs to the second generation of antihistamines, which are marketed as non-drowsy alternatives to older drugs like Benadryl. In practice, cetirizine is the most sedating drug in its class. Clinical trials show somnolence rates of 11 to 14 percent at the standard 10 mg dose. For comparison, Allegra (fexofenadine) causes drowsiness in only 1.3 to 2.2 percent of users, and Claritin (loratadine) falls in between at 4 to 8 percent.
At the higher 20 mg dose sometimes used for chronic hives, the drowsiness rate jumps to nearly 24 percent, roughly three times the rate seen with a placebo. This sedation tends to be most noticeable when you first start taking the drug or increase your dose. Some people adjust after a few days, while others feel persistently sluggish for as long as they take it. If drowsiness is a dealbreaker, switching to fexofenadine typically offers the least sedation of the three major options.
Other Common Side Effects
After drowsiness, the next most frequently reported effects in clinical trials are dry mouth (about 8 percent of users at 20 mg) and fatigue (about 7 percent). These numbers drop at the standard 10 mg dose but remain noticeably higher than placebo.
Less common effects, each occurring in 2 percent of users or fewer, include:
- Digestive issues: nausea, abdominal pain, diarrhea, constipation, and increased appetite
- Throat irritation: sore throat or pharyngitis
- Dizziness
- Headache
Most of these are mild and resolve on their own. The increased appetite effect is worth knowing about if you’ve noticed unexplained weight changes while taking Zyrtec daily, since it’s not something most people connect to an allergy pill.
Itching After Stopping Zyrtec
This is the side effect that surprises most people. After taking Zyrtec daily for an extended period (typically three months or longer), some users develop severe, widespread itching within one to five days of stopping the medication. The itching is not a return of the original allergy symptoms. It’s a rebound reaction, and it can be intense enough to interfere with sleep, work, and daily functioning.
The FDA identified 209 cases reported between 2017 and 2023, and while the agency notes this appears to be rare relative to how many people take cetirizine, the severity is notable. Of those cases, 48 involved people who described the itching as disabling, with some reporting being bed-ridden. Three cases required hospitalization. Two people reported thoughts of self-harm. The median time people had been taking the drug before this happened was about 33 months, though it occurred in people who had used it for as little as one week and as long as 23 years.
What makes this particularly frustrating is that the itching creates a cycle that’s hard to break. In 92 out of 93 cases where people restarted the medication and then tried stopping again, the itching came back. Restarting Zyrtec resolved the itching 90 percent of the time, but that obviously doesn’t solve the underlying problem. Gradually tapering the dose after restarting worked for about 38 percent of people who tried it. If you’ve been taking Zyrtec daily for several months and want to stop, reducing your dose gradually rather than quitting abruptly may lower the risk of this reaction.
Alcohol and Other Drug Interactions
Because Zyrtec already has sedating properties, combining it with alcohol amplifies both drowsiness and impaired coordination. Even one or two drinks can make you significantly more impaired than either substance alone would. The same principle applies to other sedating substances: sleep aids, anti-anxiety medications, muscle relaxants, and opioid pain medications all compound the drowsiness and can lead to dangerous levels of sedation when combined with cetirizine.
If you take any medication that makes you sleepy, check with a pharmacist before adding daily Zyrtec. The interaction isn’t subtle. It can meaningfully impair your ability to drive or operate equipment.
Higher Risk for Older Adults
Zyrtec is cleared from the body primarily through the kidneys, and kidney function naturally declines with age. In adults over 65, the drug’s half-life (the time it takes your body to eliminate half the dose) increases by about 50 percent, and clearance drops by 40 percent. That means the drug builds up to higher levels in the body, making side effects like drowsiness, dizziness, and dry mouth more likely and more pronounced.
For people with moderate kidney impairment, the changes are even more dramatic: a threefold increase in half-life and a 70 percent decrease in clearance. This means the standard 10 mg daily dose can behave more like a much higher dose in someone whose kidneys aren’t filtering efficiently. People with severe kidney problems typically need a reduced dose, and the drug is not recommended at all for those on dialysis or with end-stage kidney disease.
If you’re over 65 and noticing that Zyrtec makes you foggy or unsteady, the explanation may be pharmacological rather than just “getting older.” A lower dose or a switch to a different antihistamine could make a meaningful difference.
Children’s Zyrtec
Zyrtec is approved for children as young as 6 months in liquid form and 2 years in chewable tablets. The side effect profile in children is broadly similar to adults, with drowsiness, stomach pain, and sore throat being the most common complaints. The same FDA warning about rebound itching after discontinuation applies to pediatric formulations, so if your child has been taking Zyrtec daily for several months, a gradual taper is a reasonable approach when stopping.
Parents sometimes notice that their child becomes irritable or hyperactive on Zyrtec rather than drowsy. This paradoxical reaction is more common in young children than in adults and, while not dangerous, can be disruptive enough to warrant trying a different allergy medication.