Cetirizine vs Levocetirizine: Which Causes Less Drowsiness and Why

When you’re stuck with sneezing, itchy eyes, or a runny nose from allergies, you want relief-fast. But you don’t want to feel like you’ve been hit by a truck afterward. That’s where cetirizine and levocetirizine come in. Both are popular allergy pills you can buy over the counter. One is Zyrtec. The other is Xyzal. They look similar. They work similarly. But when it comes to drowsiness, there’s a real difference-and it matters more than you think.

What’s the Big Difference Between Them?

Cetirizine and levocetirizine aren’t just different brands-they’re chemically different. Think of cetirizine as a pair of gloves: one left, one right. Both gloves look alike, but only one fits your hand properly. Cetirizine is a mix of two mirror-image molecules: one active (levocetirizine) and one mostly inactive (dextrocetirizine). Levocetirizine is just the one glove that actually works. That’s why it’s called a third-generation antihistamine-it’s the purified version, stripped of the extra stuff.

This isn’t just chemistry class trivia. It’s why you can take 5 mg of cetirizine and get the same allergy relief as 2.5 mg of levocetirizine. The active part in both is identical. But the extra half in cetirizine? It doesn’t help with allergies. And it might be why you feel sleepy.

Which One Makes You Sleepier?

Here’s the simple answer: cetirizine causes drowsiness more often than levocetirizine.

Multiple studies back this up. One key trial with 18 healthy adults showed that levocetirizine at 2.5 mg blocked histamine just as well as cetirizine at 5 mg-but with less impact on brain activity linked to drowsiness. Another study in children with year-round allergies found both worked well, but cetirizine users reported more fatigue during the day.

Real people notice it too. On forums like Reddit and Drugs.com, users who switched from Zyrtec to Xyzal say the same thing: "I used to nap after lunch. Now I’m alert all day." One user wrote: "I work in an office. Zyrtec made me zone out during meetings. Xyzal? I didn’t even notice I was taking it."

That said, it’s not zero drowsiness with levocetirizine. About 6% of users still report feeling tired. But with cetirizine? That number jumps to 14%. And for people who are extra sensitive-like older adults or those taking other meds that affect the brain-the difference is even clearer.

Side Effects: Beyond Just Sleepiness

Both medications are generally safe. But side effects don’t stop at drowsiness. Here’s what else you might run into:

• Cetirizine: Dry mouth, headache, fatigue, sore throat, nausea. The drowsiness is the biggest complaint.
• Levocetirizine: Dry mouth, headache, tiredness-same list, but less frequent. Users report fewer cases of next-day grogginess.

Neither causes the same level of sedation as older antihistamines like diphenhydramine (Benadryl). But if you’ve ever woken up feeling like you didn’t sleep, even after taking an allergy pill at night, you’ve felt the quiet hangover of cetirizine. Levocetirizine doesn’t always avoid that-but it does reduce the odds.

One thing both share: they’re cleared by your kidneys. If you have kidney problems, your doctor might lower your dose. That’s true for both. Neither is linked to liver damage. Neither causes weight gain. Neither is addictive.

Effectiveness: Does One Work Better?

For most people, both do the same job. They stop sneezing. They calm hives. They reduce nasal itching. In clinical trials, they’re equally good at controlling symptoms of hay fever and chronic urticaria.

But here’s the twist: some people swear one works better for them. A 2022 survey of over 1,200 users found that 47% said cetirizine worked better for their symptoms. Another 38% said levocetirizine. The rest were neutral. That’s not a big gap. But it’s enough to suggest your body might respond differently to the inactive part of cetirizine-even if it doesn’t help with allergies, it might affect how your brain or gut reacts.

Doctors often say: try one for two weeks. If you’re still tired, switch. If your symptoms aren’t gone, try the other. It’s not about which is "stronger." It’s about which one your body tolerates.

Cost and Availability

Cetirizine is cheaper. A 30-day supply of 10 mg tablets costs about $13. Levocetirizine? Around $14.50. That’s not a huge difference-but if you’re paying out of pocket and you don’t have drowsiness issues, why spend more?

But if you’re a working parent, a truck driver, or someone who can’t afford to nod off during the day, that extra $1.50 might be worth it. Levocetirizine is also available in lower doses (2.5 mg), which is helpful for kids or people who need to start slow.

Both are sold without a prescription in the U.S. and the U.K. You’ll find them next to the cold medicine in any pharmacy. No special order needed.

Who Should Take Which?

Here’s a quick guide based on real-life needs:

• Choose cetirizine if: You’re on a budget, you take it at night, and you don’t feel drowsy the next day. It’s also the go-to for kids under 6-because it’s been used longer and has more pediatric dosing data.
• Choose levocetirizine if: You’re sensitive to drowsiness, you work during the day, you drive, or you’ve had bad reactions to other antihistamines. It’s also a smart pick if you’ve tried cetirizine and still feel foggy.

There’s no rule that says you have to pick one and stick with it. Many people rotate between them seasonally. Others keep both on hand-cetirizine for nights, levocetirizine for days.

What Experts Say

The American College of Allergy, Asthma, and Immunology says both are excellent choices. But they add this: "For patients who report drowsiness with cetirizine, switching to levocetirizine at half the dose often provides the same relief with less sleepiness."

Not every expert agrees that the difference is huge. Some say the sedation gap is small and mostly noticeable in people who are already prone to fatigue. But if you’re one of those people? That small gap matters a lot.

One doctor in Chicago put it this way: "I don’t push levocetirizine because it’s "better." I push it because it’s easier for people to live with."

What’s New in 2025?

There’s no major breakthrough yet, but the market is shifting. New combo pills with levocetirizine and montelukast (a nasal spray-type drug) are hitting shelves. These are for people with both allergies and asthma symptoms.

Pharmacogenomics-testing how your genes break down meds-is starting to show up in allergy clinics. In a few years, your doctor might run a quick saliva test to tell you whether you’re likely to feel drowsy on cetirizine. Until then, trial and error is still the best tool.

Bottom Line: Pick Based on Your Life, Not the Label

Levocetirizine isn’t a miracle drug. Cetirizine isn’t outdated. Both are safe, effective, and widely used. The real question isn’t which one is stronger. It’s: which one lets you live your day without feeling like you’re half-asleep?

If you’ve never tried either, start with cetirizine. It’s cheaper. If you’re fine after a week, stick with it. If you feel sluggish, switch to levocetirizine. You might not notice the difference in symptom control-but you’ll notice the difference in your energy.

And if you’re still unsure? Talk to your pharmacist. They’ve seen hundreds of people try both. They know which one works for the office worker, the new parent, the student cramming for exams. They won’t push a brand. They’ll help you find the one that fits your life.

If you’re choosing between these two, don’t overthink it. Test one. If it works and you feel fine, keep it. If you’re dragging through your day, try the other. Your body will tell you which one it prefers.