Cetirizine vs Levocetirizine: Which Causes Less Drowsiness and Why

What’s the real difference between Zyrtec and Xyzal?

If you’ve ever taken Zyrtec and felt like you were drifting off at your desk, you’re not alone. Millions of people use cetirizine and levocetirizine every day to fight allergies, but one of them makes you sleepy more often than the other. It’s not just marketing. There’s a real, science-backed reason why some people switch from Zyrtec to Xyzal-and why others stick with the cheaper option.

Both drugs work the same way: they block histamine, the chemical your body releases when it thinks pollen, dust, or pet dander is an invader. That stops the sneezing, itching, and runny nose. But here’s where it gets interesting. Cetirizine, sold as Zyrtec, is a mix of two mirror-image molecules. One of them does all the work. The other? Almost nothing. Levocetirizine, sold as Xyzal, is just that one useful molecule-cleaned up and isolated. That tiny difference changes how your brain reacts to the drug.

Why one makes you sleepy and the other doesn’t

Here’s the key: sedation isn’t about how strong the drug is. It’s about how much of it gets into your brain. Cetirizine, because it contains both the active and inactive molecules, crosses the blood-brain barrier more easily. That’s why some people feel foggy, tired, or even clumsy after taking it-even at the lowest dose. Levocetirizine, the purified version, is more selective. It sticks to histamine receptors in your nose and skin but has a harder time sneaking into your brain. That’s why it’s called a third-generation antihistamine. It’s not stronger. It’s smarter.

Studies back this up. In a 2008 trial with 18 healthy volunteers, levocetirizine at 2.5 mg blocked histamine just as well as cetirizine at 5 mg. But when researchers measured drowsiness using standard tests, people on levocetirizine were noticeably less impaired. Another study in kids with year-round allergies found both drugs worked well, but those on cetirizine reported more fatigue. That’s not a fluke. It’s built into the chemistry.

Think of it like two identical cars. One has a dirty engine. The other has been cleaned out. Both go the same speed. But the clean one runs smoother and doesn’t overheat as easily. That’s levocetirizine versus cetirizine.

Dosing: Half the pill, same results

You don’t need to take twice as much levocetirizine to get the same effect. In fact, you take half. A 2.5 mg tablet of Xyzal equals a 5 mg tablet of Zyrtec. A 5 mg Xyzal tablet equals a 10 mg Zyrtec. That’s not just convenient-it’s important. Taking less drug means less chance of side effects, even if you’re sensitive.

Most people start with 5 mg of cetirizine or 2.5 mg of levocetirizine. If symptoms aren’t controlled after a few days, you can bump up to 10 mg or 5 mg. But here’s the catch: if you’ve been on Zyrtec and feel drowsy, switching to Xyzal at half the dose often fixes it without losing symptom control. A lot of people report feeling alert for the first time in years after making the switch.

Side effects: What you’re really dealing with

Both drugs are safe for most people. But side effects aren’t the same. Cetirizine’s most common complaint? Drowsiness. Up to 14% of users report feeling sleepy, even when taking it at night. Some say it lingers into the next day-like a mental hangover. Others notice trouble concentrating at work or school.

Levocetirizine? Drowsiness happens in about 6% of users. That’s still possible, especially if you’re extra sensitive, but it’s half the rate. Other side effects like dry mouth, headache, or sore throat are nearly identical between the two. Neither causes weight gain, high blood pressure, or liver damage at normal doses. Neither is addictive. And neither should be mixed with alcohol-that’s a bad combo for anyone.

One thing to watch: if you have kidney problems, both drugs can build up in your system. Your doctor might lower your dose. That’s true for both. But if you’re healthy, you’re fine.

Real people, real experiences

Online forums are full of stories. On Reddit, one user wrote: "I was nodding off during Zoom calls on Zyrtec. Switched to Xyzal. Same allergy control. No more naps at 2 p.m." Another said: "I tried Xyzal for a month. Felt great. Then I ran out. Went back to Zyrtec. Felt like a zombie again. No contest."

But not everyone agrees. Some people say Zyrtec works better for their hives. Others say they’ve taken Zyrtec for 10 years and never felt sleepy. That’s normal. People react differently. Your body’s enzymes, your metabolism, your genetics-all of that plays a role. One person’s miracle drug is another person’s drowsiness machine.

Drugs.com reviews show levocetirizine scores slightly higher for side effects (6.7/10) than cetirizine (5.8/10). Effectiveness is nearly tied-6.5 vs 6.3. So if your main problem is feeling tired, levocetirizine has a clear edge. If cost matters more, cetirizine wins.

Price and availability: What’s worth paying for

At the pharmacy, 30 tablets of generic cetirizine 10 mg cost about $13. The same number of levocetirizine 5 mg tablets? Around $14.50. That’s not a big gap. But if you’re taking one pill a day, you’re getting two doses of cetirizine for the price of one of levocetirizine. So over time, cetirizine saves you money.

Both are available over the counter in the U.S. No prescription needed. You can find them in any drugstore, supermarket, or online. Generic versions are everywhere. The only difference? The name on the bottle. One says Zyrtec. The other says Xyzal. But inside? They’re not the same.

Who should choose which?

Choose cetirizine if:

• You’re on a tight budget
• You don’t feel drowsy with it
• You’re treating kids (it’s more commonly prescribed for children)
• You’ve tried it before and it worked fine

Choose levocetirizine if:

• You get sleepy on Zyrtec-even a little
• You need to drive, operate machinery, or stay sharp at work
• You’ve tried other antihistamines and they made you tired
• You’re willing to pay a few extra dollars for less brain fog

There’s no right or wrong. It’s personal. But if drowsiness is your biggest complaint, levocetirizine is the better option. Not because it’s stronger. Because it’s cleaner.

What if neither works?

If you’ve tried both and still feel itchy, sneezy, or congested, it might not be allergies. Or maybe you need something else. Loratadine (Claritin) and fexofenadine (Allegra) are other non-sedating options. Montelukast (Singulair) helps with nasal congestion. Some people combine antihistamines with nasal sprays. If your symptoms last more than a few weeks or get worse, talk to an allergist. They can test what you’re really reacting to-and find a better plan.

Final thought: It’s not about the brand. It’s about your body.

Neither drug is perfect. Neither is dangerous. But if you’re choosing between them, pay attention to how you feel-not just the price tag. Try one for two weeks. Then switch to the other for two weeks. Keep a simple note: "Did I feel tired? Did my allergies improve?" That’s all you need. Your body will tell you which one is right for you.