Cetirizine vs Levocetirizine: Which Causes Less Drowsiness and Why

What’s the real difference between Zyrtec and Xyzal?

If you’ve ever taken an allergy pill and felt like you needed a nap by noon, you’re not alone. Cetirizine (Zyrtec) and levocetirizine (Xyzal) are both common choices for runny noses, itchy eyes, and hives. They work similarly. They’re both taken once a day. But one of them leaves more people feeling foggy - and that’s the key difference. Cetirizine is a racemic mixture. That means it’s made of two mirror-image molecules: one active (levocetirizine) and one mostly inactive (dextrocetirizine). Think of it like two keys that look almost identical - one fits the lock perfectly, the other barely turns. Levocetirizine is just the good key. It’s the purified, active part of cetirizine. That’s why you take half the dose: 2.5 mg of levocetirizine works about the same as 5 mg of cetirizine.

Why does sedation matter?

You don’t want to feel sleepy when you’re driving, working, or picking up your kids. That’s why the sedation difference between these two drugs isn’t just a footnote - it’s a deciding factor for many people. Studies show cetirizine causes drowsiness in about 14% of users at the standard 10 mg dose. That number drops to around 6% with levocetirizine at 5 mg. That’s not a huge gap, but for someone who’s sensitive, it’s enough to notice. One 2008 study in healthy volunteers found that levocetirizine blocked histamine just as well as cetirizine - but with less brain penetration. Less brain penetration means less drowsiness. The FDA labels for both drugs say they’re "non-sedating," but that’s a technical term. It doesn’t mean zero sleepiness. It means less than older antihistamines like diphenhydramine (Benadryl). Still, if you’ve ever had to cancel a meeting because you couldn’t keep your eyes open after taking Zyrtec, you know it’s real.

How do they compare in real life?

Real-world feedback tells a clear story. On Drugs.com, users rated levocetirizine higher for side effects: 6.7 out of 10, compared to 5.8 for cetirizine. On Reddit, people say things like, "I switched from Zyrtec to Xyzal and suddenly I could focus at work," or, "Xyzal doesn’t make me feel like I’ve had one too many glasses of wine." But it’s not universal. Some people swear by Zyrtec. One WebMD reviewer wrote, "I’ve tried both. Zyrtec works better for me, and I don’t get sleepy." That’s important. Everyone’s body reacts differently. Genetics, liver function, and even what you eat can change how you respond.

Effectiveness: Is one better?

Both drugs are effective. For allergic rhinitis and chronic hives, they’re equally good at reducing symptoms. A 2009 study in children with year-round allergies found cetirizine had a slight edge in symptom relief. But other studies, including the one from 2008, show they’re nearly identical. The truth? If you’re not sleepy on cetirizine, there’s no reason to switch. But if you’re struggling with drowsiness, levocetirizine is a smart next step - especially since you can get the same relief with half the dose.

Dosing: What should you take?

Don’t confuse the doses. 5 mg of cetirizine = 2.5 mg of levocetirizine. 10 mg of cetirizine = 5 mg of levocetirizine. Most adults start with: - Cetirizine: 5-10 mg once daily - Levocetirizine: 2.5-5 mg once daily Kids under 6 usually get lower doses based on weight. Always check the label or ask your pharmacist. Neither drug is approved for children under 2 without a doctor’s direction.

Cost and availability

Cetirizine is cheaper. A 30-day supply of 10 mg tablets runs about $13. Levocetirizine (5 mg) is closer to $14.50. That’s not a big gap - but if you’re paying out of pocket, every dollar counts. Both are available over-the-counter in the U.S. since 2007 (cetirizine) and 2017 (levocetirizine). You’ll find them next to the cold medicine aisle, no prescription needed.

Side effects: What else should you watch for?

Besides drowsiness, both drugs can cause: - Dry mouth - Headache - Sore throat - Fatigue - Nausea Serious side effects are rare. But if you have kidney problems, your doctor may lower your dose. Both drugs are cleared by the kidneys. If your kidney function is reduced, the medicine can build up and increase side effects. Neither drug interacts badly with most foods or alcohol, but mixing them with other sedatives - like sleep aids or painkillers with tramadol - can make you extra sleepy. Use caution.

Who should pick which?

Cetirizine is the better choice if: - You’re on a tight budget - You don’t get drowsy from it - You’re treating a child (it’s more commonly prescribed for kids) - You want the most widely tested option Levocetirizine is better if: - You get sleepy on cetirizine - You need to stay alert for work, driving, or school - You’ve tried other antihistamines and they didn’t work well - You’re willing to pay a little more for slightly fewer side effects

What if neither works?

If you’re still sneezing, itching, or congested after trying both, you’re not alone. About 1 in 5 people don’t respond fully to either drug. That’s when you might need to try: - Loratadine (Claritin) - Fexofenadine (Allegra) - A nasal steroid spray like fluticasone - Or a combination pill like levocetirizine with montelukast (a newer option)

What’s next for these drugs?

Researchers are looking at personalized dosing. Soon, we might test your genes to see how fast you break down antihistamines - and then pick the right drug and dose for you. For now, the best advice is simple: try one for two weeks. If you’re still tired, switch to the other. Track your symptoms and sleepiness. You’ll learn what your body needs.

Bottom line

Cetirizine and levocetirizine are both safe, effective, and widely used. The main difference? Sedation. Levocetirizine is less likely to make you drowsy - and you take half the dose to get the same results. If drowsiness is a problem for you, it’s worth trying. If you’re fine on cetirizine, stick with it. There’s no need to pay more unless you need the upgrade.