In this article:

How long does it take Zyrtec to work?
How long does Zyrtec last?
Interactions and safety considerations when taking Zyrtec

In this article

How long does it take Zyrtec to work?
How long does Zyrtec last?
Interactions and safety considerations when taking Zyrtec

Runny nose. Itchy eyes. A seemingly unending chain of sneezes in the middle of a conversation. It’s the plight of seasonal allergies, allergic rhinitis, and hay fever, and the prevailing question among anyone who’s taken Zyrtec is, “How fast will it work?” Good news: this brand-name allergy medicine blocks histamines that cause symptoms and may start working within an hour—but its efficiency and duration depend on various factors.

Here are all the crucial details about how long Zyrtec takes, how long it lasts, and who can take it.

How long does it take Zyrtec to work?

Whether sounding stuffy on the phone or unable to taste your lunch, common allergy symptoms from pollen, mold, dust, or other sources can dampen the day. Fortunately, most people don’t have to wait too long for sweet relief. “Antihistamines, such as Zyrtec, are fast-acting allergy medications,” says Dr. Luke Prest, MD, pediatrician and author at ChoosingTherapy. “Zyrtec usually takes effect within 30 to 60 minutes.”

According to the National Institutes of Health website, Zyrtec reaches peak blood concentration at around 60 minutes, so most patients feel the most significant improvement then. This study backs that up, showing that most people reported effects within one hour. Zyrtec-D, which includes a decongestant, has a similar timeframe. However, according to Dr. Ila Dayananda, MD, MPH, Chief Medical Officer of Oula Health, “the onset of action may vary slightly depending on individual factors such as metabolism, symptom severity, and other medications or medical conditions.”

According to the United States Food and Drug Administration (FDA), taking Zyrtec with food won’t alter its effectiveness, but it may delay its peak concentration by 1.7 hours.

How long does Zyrtec last?

How fast it works is one thing, but how long it works is equally important. The FDA says Zyrtec’s effects typically last around 24 hours, and according to studies, its elimination half-life is around 8.3 hours. After taking the recommended dose, most people won’t take another until the next day.

Zyrtec-D, however, is a little different. First off, it only lasts 12 hours instead of 24. That’s primarily because it includes pseudoephedrine for nasal congestion. While the FDA says the half-life of the cetirizine hydrochloride (Zyrtec’s active ingredient) in this formulation is 7.9 hours, the half-life of pseudoephedrine is closer to six hours. So, to sustain the drug’s congestion relief, you have to take it twice a day.

“Taking the appropriate dosage of Zyrtec, as directed by your healthcare provider or as indicated on the product label, is essential for optimal effectiveness,” says Dr. Dayananda. “The timing is also important, as taking Zyrtec at recommended intervals is necessary to maintain consistent levels in your bloodstream.”

Is it better to take Zyrtec at night or in the morning?

Short answer: Either. Slightly longer answer: It depends on the circumstances.

Zyrtec is a second-generation antihistamine medication, so it’s much less likely to cause drowsiness than first-generation antihistamines like Benadryl, which may pass the blood-brain barrier, according to the American Academy of Allergy, Asthma, & Immunology site. So, if you take it when you wake up, there’s a good chance it won’t affect your daily activities. That said, in clinical trials, 14% of people taking Zyrtec 10mg reported experiencing sleepiness. So, it is important to see how it affects you before taking it first thing in the morning.

But for people who don’t experience drowsiness, it comes down to when allergies hit the hardest. People who get them primarily during the day might want to take it in the morning, while others can take it before bed if they experience symptoms. For standard 10mg Zyrtec, children and adults over six should take one tablet daily. Adults ages 65 and older should reduce that dosage to 5mg. For Zyrtec-D, take one tablet every 12 hours and no more than two per day. Zyrtec does not make a nasal spray.

How long do I take Zyrtec?

Allergies are often a temporary inconvenience. They last for a particular season or during contact with a particular allergen and then subside. In most cases, people simply take Zyrtec until their allergies go away. “It is unlikely to cause harm if taken long-term,” says Dr. Lisa Kipper, Pharm.D., director of experiential education at Marshall B. Ketchum University. “But it should only be taken for as long as you have symptoms, usually less than seven days.”

That said, people who take Zyrtec for a long time and then stop suddenly may experience symptoms of withdrawal. Zyrtec withdrawal may cause itching, hives, vivid dreams, and headaches, which may last for around a week. Not everyone who takes Zyrtec long-term will get a withdrawal, but it’s a possibility.

A study published in 2015 linked long-term Benadryl use with a higher risk of dementia, which caused some concern. Second-generation OTC oral antihistamines like Zyrtec and Claritin don’t necessarily pose the same risk, but there’s not a ton of research on it yet.

Interactions and safety considerations when taking Zyrtec

Zyrtec is available over the counter—but that doesn’t mean it’s a good option for everyone. While it has fewer side effects than older antihistamines and some prescription drugs, it still carries the risk of side effects, interactions, and precautions.

Side effects

“Zyrtec is generally well tolerated with few to no side effects,” Dr. Prest says. “But the most

common side effects of Zyrtec include drowsiness, fatigue, dizziness, and dry mouth.” Beyond that, other potential side effects include:

Headache
Sore throat
Abdominal pain
Cough
Nosebleeds
Diarrhea
Nausea
Vomiting
Insomnia

Most of these adverse reactions are relatively harmless, but in rare cases, Zyrtec may cause low blood pressure, dizziness, a racing heart, and fainting. A serious allergic reaction, albeit rare, is also possible and often manifests in a skin rash, itchiness, dizziness, and difficulty breathing. Anyone who experiences one or more of these serious side effects should seek medical help immediately.

Note: Considering that Zyrtec blocks histamine release, an immune response, some might think it also suppresses the immune system, but studies have shown that this isn’t the case.

Drug interactions

Anyone taking Zyrtec should avoid drinking alcohol or taking other central nervous system depressants, such as benzodiazepines or opioids, according to the NIH. These drugs may compound the drowsiness that some people feel while taking Zyrtec. Other drugs that may cause drowsiness include tranquilizers, gabapentin, general anesthetics, and cannabis.

Per Dr. Dayananda, “Interactions with other medications or substances may also affect the efficacy of Zyrtec, so it’s imperative to inform your doctor about any medications, supplements, and herbal remedies you are taking before starting Zyrtec.”

Other precautions

There’s no concrete evidence showing that Zyrtec is harmful to pregnant women, but the FDA’s drug information still says, “Zyrtec should be used in pregnancy only if needed.” It also gets excreted in human milk, and although the National Institute of Child Health and Human Development says small, occasional doses during breastfeeding are acceptable, it’s generally not recommended.

“Those with liver or kidney problems may require lower doses of Zyrtec to prevent side effects,” says Dr. Prest. The liver helps filter it out, and if it’s not functioning at full capacity, it may cause higher levels of the drug for longer periods. Consult a healthcare provider before taking Zyrtec if you have liver disease or kidney disease. Dr. Kipper adds: “If you have asthma, glaucoma, enlarged prostate, heart disease, diabetes, or thyroid disease, talk to your pharmacist or doctor.”

Finally, adults older than 65 are more likely to experience side effects while taking Zyrtec. This doesn’t mean older adults can’t take it. Rather, the drug’s dosage guide suggests that they speak with a healthcare provider beforehand.

How long does it take for Zyrtec to work?