Cetirizine vs Levocetirizine: Which Causes Less Drowsiness and Why

When you’re fighting allergies, the last thing you want is to feel like you’ve been hit by a truck after taking your pill. Two of the most common over-the-counter antihistamines-cetirizine (Zyrtec) and levocetirizine (Xyzal)-do the same job: block histamine to stop sneezing, itching, and runny nose. But one of them leaves you feeling clearer-headed. And it’s not just marketing.

They’re not as different as you think

Cetirizine and levocetirizine are like twins born from the same chemical parents. Cetirizine is a racemic mixture-meaning it’s made of two mirror-image molecules: one active, one nearly useless. The active one? That’s levocetirizine. The other half? Dextrocetirizine. Think of it like a pair of gloves: one fits perfectly (levocetirizine), the other doesn’t do much (dextrocetirizine). When you take a 10 mg pill of Zyrtec, you’re getting 5 mg of the good stuff and 5 mg of filler that doesn’t help with allergies but might contribute to side effects.

Levocetirizine, sold as Xyzal, is just the clean version-the 5 mg of pure active ingredient. That’s why you only need half the dose to get the same allergy relief. Studies show 2.5 mg of levocetirizine works just as well as 5 mg of cetirizine at blocking histamine. In fact, levocetirizine binds to H1 receptors about 30 times more tightly than its inactive twin, according to research published in PubMed.

The real difference: drowsiness

Here’s where things get practical. Both drugs are labeled as "non-sedating," and compared to old-school antihistamines like diphenhydramine (Benadryl), they’re right. But "non-sedating" doesn’t mean "no sedation." A lot of people still feel a little foggy, especially in the first few days or if they’re sensitive.

Multiple clinical trials point to levocetirizine having a slight edge. One study with 18 healthy adults found that while both drugs blocked histamine equally well, levocetirizine caused less subjective drowsiness. Another study in children with year-round allergies showed both worked, but those on levocetirizine reported fewer reports of fatigue. Real-world feedback backs this up. On Drugs.com, users give levocetirizine a 6.7/10 for side effects versus 5.8/10 for cetirizine. Common comments? "Xyzal doesn’t knock me out like Zyrtec did." "I can actually focus at work now."

But here’s the catch: not everyone feels it. Some people swear Zyrtec doesn’t make them sleepy at all. Others say Xyzal still leaves them tired. Why? Because metabolism, body weight, and genetics play a role. One person’s "non-drowsy" is another’s "nap time."

How much less drowsy is it, really?

Let’s be clear: the difference isn’t huge. If you’re a healthy adult with no history of drowsiness from allergy meds, you might not notice a thing. But if you’ve ever felt groggy after Zyrtec-especially in the afternoon or the next morning-switching to Xyzal could make a real difference.

The American College of Allergy, Asthma, and Immunology says this: if cetirizine makes you drowsy, try levocetirizine at half the dose. Many patients report the same symptom control without the fog. That’s not magic-it’s pharmacology. The inactive half of cetirizine might be crossing the blood-brain barrier more easily than levocetirizine alone, causing mild CNS effects. Levocetirizine’s cleaner structure may reduce that.

A 2022 survey of 1,500 allergy sufferers found that 58% of working adults who prioritized daytime alertness chose levocetirizine over cetirizine. That’s not because it’s stronger. It’s because it’s cleaner.

Side effects: what else should you watch for?

Beyond drowsiness, both drugs have nearly identical side effect profiles. The most common ones:

• Headache
• Sore throat
• Dry mouth
• Fatigue
• Nausea

Neither causes significant weight gain, high blood pressure, or heart issues-unlike some older antihistamines. Both are cleared mostly by the kidneys, so if you have kidney problems, your doctor may lower your dose. Neither should be mixed with alcohol, as that can boost drowsiness.

One thing to remember: both are safe for kids over 2 years old. Cetirizine is more commonly prescribed for children, partly because it’s cheaper and available in liquid form. But levocetirizine syrup is also approved for kids as young as 6 months. If your child gets sleepy on Zyrtec, switching to Xyzal might help.

Cost and availability

Cetirizine is cheaper. A 30-day supply of 10 mg tablets runs about $13. Levocetirizine? Around $14.50. That’s not a big gap, especially if you’re on a generic. But if you’re paying full price without insurance, every dollar counts.

Both are available over-the-counter in the U.S. since 2007 (cetirizine) and 2017 (levocetirizine). You won’t need a prescription. You’ll find them next to the cold and flu meds in any pharmacy. Some stores even sell them in blister packs labeled "Daytime Allergy Relief"-a hint that the manufacturer knows which one people prefer for staying alert.

Who should pick which?

Here’s a simple guide:

• Choose cetirizine (Zyrtec) if: you’re on a tight budget, you’ve never felt drowsy on it, or you’re treating a child under 6 (where liquid forms are more common).
• Choose levocetirizine (Xyzal) if: you’ve felt sleepy on Zyrtec, you need to drive, work, or study after taking it, or you want the cleanest version of the drug with no extra inactive ingredients.

There’s no "better" drug-just the better drug for you.

What if neither works?

If you’ve tried both and still feel tired or your allergies aren’t controlled, it’s time to look beyond antihistamines. Some people respond better to fexofenadine (Allegra) or loratadine (Claritin). Others need a nasal spray like fluticasone. If your symptoms are year-round and severe, your doctor might suggest adding a leukotriene blocker like montelukast (Singulair)-a combo now available in one pill with levocetirizine.

Also, consider environmental triggers. If you’re constantly exposed to dust mites, pollen, or pet dander, medication alone won’t fix it. Air purifiers, HEPA filters, and allergy-proof bedding can reduce your need for pills.

Final thought: try both

The best way to know which one works for you? Try each for 7 to 10 days. Take cetirizine 5 mg daily for a week. Note how you feel-especially after lunch. Then switch to levocetirizine 2.5 mg for another week. Keep a quick journal: "Did I nap after dinner? Could I focus at work? Did my eyes stop itching?" You might be surprised. Some people think they’re "fine" on Zyrtec until they try Xyzal and realize they’ve been living with low-grade drowsiness for years.

Both drugs are safe, effective, and widely used. The choice isn’t about science alone-it’s about how your body responds. And that’s something no study can predict for you.