What Is in Zyrtec? Active & Inactive Ingredients

The active ingredient in every Zyrtec product is cetirizine hydrochloride (sometimes labeled cetirizine dihydrochloride), a second-generation antihistamine that blocks histamine H1 receptors. A standard Zyrtec tablet contains 10 mg of cetirizine along with a handful of inactive ingredients that hold the pill together, coat it, and give it its shape. But not all Zyrtec products are identical, and the inactive ingredient lists vary across tablets, liquid gels, dissolvable forms, children’s syrup, and the decongestant combo product Zyrtec-D.

How Cetirizine Works

Cetirizine belongs to the second-generation class of antihistamines, meaning it was designed to relieve allergy symptoms with less drowsiness than older options. When your body encounters an allergen like pollen or pet dander, immune cells release histamine, which binds to H1 receptors and triggers the familiar cascade of sneezing, itching, runny nose, and watery eyes. Cetirizine parks itself on those H1 receptors and blocks histamine from binding, which dials down the allergic response.

What makes cetirizine stand out from some other antihistamines is an additional anti-inflammatory property. Research has shown it has a potent inhibitory action against the infiltration of eosinophils, a type of white blood cell involved in prolonged allergic inflammation, into affected tissue. At the time of its introduction, no other antihistamine had been reported to share this antieosinophilic characteristic.¹ In people with allergic asthma, cetirizine was found to inhibit the recruitment of inflammatory cells, mainly eosinophils, triggered by allergen exposure, even when it did not fully block the immediate airway response itself.² This dual action helps explain why cetirizine can be effective for both acute symptoms and conditions with ongoing allergic inflammation, like chronic hives.

Inactive Ingredients in Standard Zyrtec Tablets

The 10 mg film-coated tablet, the most widely purchased version, contains inactive ingredients whose job is structural and cosmetic rather than therapeutic. The typical list for the original Zyrtec tablet includes colloidal silicon dioxide, croscarmellose sodium, hypromellose, lactose monohydrate, magnesium stearate, microcrystalline cellulose, polyethylene glycol, and titanium dioxide. Some formulations also contain iron oxide for coloring.

Each of those has a specific role. Microcrystalline cellulose and lactose monohydrate act as fillers that bulk up the tablet to a manageable size. Croscarmellose sodium is a disintegrant that helps the tablet break apart in your stomach so the cetirizine can dissolve. Magnesium stearate prevents the powder from sticking to manufacturing equipment. Hypromellose and polyethylene glycol form the smooth coating that makes the tablet easier to swallow, while titanium dioxide gives it a white appearance. Colloidal silicon dioxide improves the powder’s flow properties during production.

If you pick up the liquid gel capsule version instead, the inactive ingredient list shifts. Liquid gels typically use gelatin for the capsule shell, glycerin, pharmaceutical ink for the printed label, polyethylene glycol as a solvent, purified water, sodium hydroxide for pH adjustment, and sorbitol or sorbitan. The active ingredient and its dose remain exactly the same; only the delivery vehicle changes.

Zyrtec-D and What Makes It Different

Zyrtec-D adds a second active ingredient: pseudoephedrine hydrochloride, a nasal decongestant. While cetirizine handles the histamine-driven symptoms like itching and sneezing, pseudoephedrine constricts swollen blood vessels in the nasal passages to relieve stuffiness. The combination tablet contains 5 mg of cetirizine dihydrochloride for immediate release and 120 mg of pseudoephedrine hydrochloride for extended release, taken twice daily. A once-daily version has also been developed containing 10 mg of cetirizine for immediate release and 240 mg of pseudoephedrine for extended release.³

The tablet design for Zyrtec-D is more complex than regular Zyrtec. The cetirizine sits in an outer layer that dissolves quickly, while the pseudoephedrine is contained in a core surrounded by a semipermeable membrane of cellulose acetate and polyethylene glycol that controls how slowly the decongestant releases over the day.³ This is why you should not crush or chew a Zyrtec-D tablet: doing so would dump the entire decongestant dose at once instead of spreading it out.

Because pseudoephedrine can raise blood pressure and heart rate, Zyrtec-D is sold behind the pharmacy counter in the United States (you do not need a prescription, but you do need to ask the pharmacist and show ID). People with high blood pressure, heart disease, thyroid problems, or glaucoma should talk to a doctor before taking it. The inactive ingredient list for Zyrtec-D differs from the standard tablet due to the layered extended-release design and includes additional polymers and coatings needed to control the pseudoephedrine release rate.

Children’s Zyrtec Formulations

Children’s Zyrtec comes as a grape- or bubble-gum-flavored syrup and as chewable tablets, both containing cetirizine hydrochloride at lower concentrations suited for pediatric dosing. The syrup is typically 1 mg per mL, so parents can measure doses precisely. For children ages 2 to 5, the usual dose is 2.5 mg (half a teaspoon) once daily, sometimes increased to 5 mg depending on the child’s response. Children 6 and older generally take 5 to 10 mg daily.

The inactive ingredients in the syrup include flavoring agents, sweeteners like sucrose or sorbitol, glycerin, propylene glycol, sodium acetate, acetic acid, methylparaben (a preservative), and purified water. Some versions use grape flavoring with artificial colors. The chewable tablets have their own inactive list that typically includes mannitol, citric acid, flavoring, aspartame (a consideration for families managing phenylketonuria), and coloring agents.

The variety of dyes and sweeteners in children’s formulations is worth noting because these are the ingredients most likely to cause concern for parents managing a child’s dietary restrictions or sensitivities.

Ingredient Sensitivities and Dye Concerns

Most people tolerate Zyrtec’s inactive ingredients without any issue, but a few components deserve attention. Lactose monohydrate is present in the standard tablet, which can be relevant for people with severe lactose intolerance. The amount is small, usually well under a gram, and most lactose-intolerant people handle it fine, but individuals with extreme sensitivity or a true milk allergy should be aware of it. Liquid gel capsules skip the lactose but contain gelatin, which rules them out for strict vegetarians or vegans.

Artificial dyes in children’s formulations and some coated tablets are another consideration. Color additives are widespread in food and medications, and certain individuals can have allergic hypersensitivity reactions to them, ranging from mild itching or rash to, in rare severe cases, throat closure.⁴ If you have a known sensitivity to specific dyes like FD&C Yellow No. 6 or FD&C Red No. 40, check the inactive ingredient list on the specific Zyrtec product you are buying, because different formulations use different colorants. Dye-free versions of children’s Zyrtec are available for families who want to avoid this issue entirely.

Aspartame in chewable tablets is safe for the general population but is a concern for individuals with phenylketonuria (PKU), a rare metabolic condition. Products containing aspartame are required to carry a PKU warning on the label.

Brand-Name Zyrtec vs. Generic Cetirizine

Zyrtec went off patent years ago, and generic cetirizine hydrochloride is now available from dozens of manufacturers at a fraction of the brand-name price. The active ingredient is identical. The inactive ingredients often differ slightly because each manufacturer uses its own formulation for fillers, binders, coatings, and colors, but these differences do not affect how well the drug works.

For a generic to be approved, it must demonstrate bioequivalence to the brand-name product, meaning the body absorbs it at essentially the same rate and to the same extent. A bioequivalence study comparing two cetirizine tablet formulations found that the confidence intervals for peak blood concentration and total drug exposure fell well within the acceptable range defined by both European and U.S. FDA guidelines, confirming the two could be prescribed interchangeably.⁵ Another pharmacokinetic study of cetirizine products similarly found that key absorption parameters for test and reference formulations were in good agreement with previously published data.⁶

In practical terms, this means the store-brand cetirizine from any major pharmacy chain will give you the same allergy relief as the Zyrtec box sitting next to it on the shelf. The one caveat: if you have a sensitivity to a specific inactive ingredient, compare the inactive ingredient lists, because the generic may use a different dye, filler, or coating than the brand. The active ingredient and its clinical effect, though, are the same.

Why Zyrtec Can Still Cause Drowsiness

Second-generation antihistamines like cetirizine were marketed as “non-drowsy,” and they do cause less sedation than older first-generation drugs like diphenhydramine (Benadryl). But cetirizine is the most sedating of the second-generation group, and drowsiness remains its most commonly reported side effect. Research has shown that all antihistamines, including second-generation ones, cross the blood-brain barrier to some degree.⁷ The distinction between generations is one of degree, not an absolute barrier. Cetirizine crosses less readily than diphenhydramine but more readily than some of its second-generation peers like loratadine (Claritin) or fexofenadine (Allegra).

This matters when you are choosing which product to buy. If drowsiness is a concern for you, such as before driving or operating machinery, you might prefer a different second-generation antihistamine. If you take Zyrtec at bedtime and the mild sedation actually helps you sleep, that can be a side benefit. The drowsiness effect tends to diminish after the first few days of regular use for many people, but not everyone.

Drug Interactions to Be Aware Of

Cetirizine has relatively few drug interactions compared to many other medications, which is one reason it is sold over the counter. It is not heavily metabolized by the liver enzymes that cause the most common drug interactions. However, researchers have found that cetirizine can inhibit P-glycoprotein, a transporter protein that pumps certain drugs out of cells and plays a role in how the body absorbs and eliminates some medications. In laboratory and intestinal studies, the effective permeability of digoxin, a heart medication that relies on P-glycoprotein for clearance, increased in a dose-dependent manner in the presence of cetirizine.⁸ At the highest tested cetirizine concentration, digoxin absorption roughly doubled and a half compared to control conditions.⁸

This does not mean cetirizine is dangerous to combine with other medications, but it does mean that people taking drugs with a narrow therapeutic window that depend on P-glycoprotein transport should mention their antihistamine use to a pharmacist or prescriber. Digoxin is the most commonly cited example, but other P-glycoprotein substrates include certain immunosuppressants and some chemotherapy agents.

Alcohol and other central nervous system depressants can amplify cetirizine’s sedating effect. This is not unique to cetirizine but is worth knowing because people sometimes forget that an over-the-counter allergy pill can interact with a glass of wine. The sedation from cetirizine at standard doses is mild on its own, but stacking it with alcohol or a sleep aid can tip the balance toward impairment.

Cetirizine in Veterinary Medicine

If you have a cat or dog with allergies, your vet may have recommended cetirizine, and you might have noticed the ingredient list is the same as the human product. Veterinarians sometimes prescribe human cetirizine tablets or syrup for pets because no veterinary-specific cetirizine formulation exists for most companion animals. A clinical trial in cats with allergic skin disease gave cetirizine at 5 mg per cat once daily and found that itching was reduced in about 41% of the cats treated, with the antipruritic effect being repeatable and sustainable over time.⁹

The important caution here involves inactive ingredients, not the cetirizine itself. Xylitol, an artificial sweetener found in some generic cetirizine formulations and other sugar-free medications, is extremely toxic to dogs and can cause a rapid, life-threatening drop in blood sugar and liver failure. If your vet recommends cetirizine for your dog, always verify that the product you are using does not contain xylitol. Cats are less susceptible to xylitol toxicity, but checking the ingredient list is still good practice. Children’s liquid formulations may also contain propylene glycol or other additives that are fine for humans but problematic for certain animals, so always use the specific product your vet recommends rather than grabbing whatever is in your medicine cabinet.

Reading the Label Yourself

Over-the-counter drug packaging in the United States follows a standardized “Drug Facts” format mandated by the FDA. The active ingredient and its amount per dose appear at the very top, followed by the purpose (antihistamine), uses, warnings, directions, and finally the inactive ingredients listed alphabetically. This format is the same whether you pick up the brand-name Zyrtec box or a store-brand generic.

One thing the label will not tell you is why certain inactive ingredients are there. Manufacturers are required to list them but not to explain their function. If you are trying to avoid a specific substance, the alphabetical list can be a wall of unfamiliar chemical names. A useful shortcut: if you have a known sensitivity, photograph the inactive ingredient panel and show it to your pharmacist. They can quickly scan for the substance you are avoiding and suggest an alternative formulation if needed. The liquid gels, standard tablets, chewable tablets, and dissolve tabs all have different inactive ingredient profiles, so switching form factor is often enough to sidestep a problematic additive without switching your active ingredient at all.