Most allergy medicines contain one of a few types of active ingredients: antihistamines, decongestants, corticosteroids, or leukotriene blockers. Many products combine two or more of these. Beyond the active ingredients, allergy pills also contain inactive fillers, binders, dyes, and sugars that occasionally matter more than you’d expect.
Antihistamines: The Most Common Ingredient
The ingredient you’ll find in the majority of allergy pills is an antihistamine. When you encounter pollen, pet dander, or dust mites, your immune system releases a chemical called histamine. Histamine latches onto receptors throughout your body, triggering the familiar cascade of sneezing, itching, watery eyes, and a runny nose. Antihistamines work by stabilizing those receptors in an inactive state, essentially preventing histamine from flipping the switch. This reduces leaking from small blood vessels (which causes swelling and congestion), calms irritated nerve fibers in your skin and nasal passages (which causes itching), and relaxes tightened airways.
There are two generations of antihistamines, and the difference matters for your daily life.
Older (First-Generation) Antihistamines
These include diphenhydramine (the active ingredient in Benadryl) and chlorpheniramine (found in Chlor-Trimeton). They work well against allergy symptoms, but they cross easily into the brain, which is why they cause drowsiness, dry mouth, and sluggish thinking. Diphenhydramine is so sedating that it’s also the active ingredient in many over-the-counter sleep aids like ZzzQuil. Another first-generation antihistamine, doxylamine, is sold as the sleep aid Unisom. These older formulas typically wear off in four to six hours, so they require multiple doses per day.
Newer (Second-Generation) Antihistamines
The ingredients in modern allergy pills were specifically designed to have a harder time crossing into the brain. That’s why they’re marketed as “non-drowsy.” The most common ones you’ll see on pharmacy shelves are:
- Cetirizine (Zyrtec)
- Loratadine (Claritin)
- Fexofenadine (Allegra)
- Levocetirizine (Xyzal)
All four are available without a prescription and last a full 24 hours per dose. Cetirizine and levocetirizine can still cause mild drowsiness in some people, while fexofenadine is generally the least sedating of the group.
Decongestants: Shrinking Swollen Nasal Passages
If your allergy medicine has a “D” after the brand name (like Zyrtec-D or Allegra-D), it contains a decongestant alongside the antihistamine. The most effective over-the-counter decongestant ingredient is pseudoephedrine. It narrows blood vessels in your nasal passages, reducing swelling so you can breathe more easily. Because pseudoephedrine can be used to manufacture methamphetamine, it’s kept behind the pharmacy counter in the U.S., and you’ll need to show ID to buy it.
For years, the alternative was phenylephrine, which sat on open shelves in dozens of cold and allergy products. That changed after the FDA reviewed the available data and found that oral phenylephrine simply doesn’t work as a nasal decongestant at recommended doses. An FDA advisory committee unanimously agreed with that conclusion, and the agency has proposed removing it from over-the-counter products. Phenylephrine in nasal spray form still works because it’s applied directly to the tissue, but the pills and liquid versions are ineffective. If you’ve been taking an allergy combo product with oral phenylephrine and it hasn’t helped your congestion, this is likely why.
Nasal Corticosteroids: Reducing Inflammation
Steroid nasal sprays are some of the most effective allergy treatments available, and several are now sold over the counter. The active ingredient is a synthetic corticosteroid, a lab-made version of hormones your body naturally produces. These ingredients dial down inflammation across the board, reducing congestion, sneezing, itching, and runny nose all at once.
The most common active ingredients in this category are:
- Fluticasone propionate (Flonase Allergy Relief), delivering 50 micrograms per spray
- Fluticasone furoate (Flonase Sensimist)
- Triamcinolone acetonide (Nasacort)
- Budesonide (Rhinocort)
- Mometasone furoate (Nasonex 24HR), which switched to over-the-counter status in 2022
Unlike decongestant sprays, which lose effectiveness after a few days, corticosteroid sprays are designed for daily use throughout allergy season. They typically take a few days of consistent use before you feel the full benefit. The doses are tiny and act locally in the nose, so they don’t produce the side effects associated with oral steroids.
Leukotriene Blockers: A Prescription Option
Not all allergy medicines target histamine. Your immune system also produces chemicals called leukotrienes during an allergic reaction. These are released from mast cells and other immune cells after allergen exposure and contribute to nasal congestion and airway tightening. Montelukast (sold as Singulair) blocks the receptor that leukotrienes bind to, reducing symptoms of both allergic rhinitis and asthma. It’s available by prescription only, and it’s typically reserved for people whose allergies don’t respond well to antihistamines or nasal sprays alone.
Mast Cell Stabilizers: Preventing the Reaction
Most allergy ingredients work after your immune system has already released its chemical signals. Mast cell stabilizers take a different approach: they stop the release from happening in the first place. The main ingredient in this category is cromolyn sodium, available as the nasal spray NasalCrom. It prevents mast cells from dumping histamine and leukotrienes into surrounding tissue. Cromolyn has no antihistamine, decongestant, or steroid activity. It works purely by keeping the mast cell stable. The trade-off is that you need to start using it before symptoms appear, and it requires multiple doses per day to maintain its effect.
Eye Drops for Allergies
Allergy eye drops contain their own set of active ingredients. Older formulations use antihistamines or decongestants, but the most effective modern drops use a dual-action ingredient called olopatadine (sold as Pataday). Olopatadine both blocks histamine receptors and stabilizes mast cells, tackling itchy, red, watery eyes from two directions. Several strengths are now available over the counter, with the once-daily version using a 0.2% concentration. Azelastine, another antihistamine, is available as both a nasal spray (Astepro) and eye drops, and both switched to OTC status in recent years.
Inactive Ingredients That Aren’t Always Harmless
Every allergy tablet contains more than just its active ingredient. The rest of the pill is made up of inactive ingredients: binders that hold the tablet together, fillers that give it bulk, coatings that help you swallow it, and dyes that give it a recognizable color. Common examples include magnesium stearate, microcrystalline cellulose, and titanium dioxide.
A study from MIT found that roughly 93 percent of medications contain at least one potential allergen in their inactive ingredients, including lactose, chemical dyes like FD&C Yellow No. 5, and occasionally peanut oil. About 55 percent of medications contain FODMAP sugars, which can cause digestive problems for people with sensitivities. Some formulations even contain trace amounts of gluten. For most people, these tiny amounts are irrelevant. But if you have a diagnosed lactose intolerance, celiac disease, or a dye sensitivity, it’s worth checking the full ingredient list on the box or asking your pharmacist for an alternative formulation.
How Combination Products Stack Up
Many products on pharmacy shelves combine two or more active ingredient types in a single pill or liquid. A typical combination pairs an antihistamine with a decongestant (like cetirizine plus pseudoephedrine in Zyrtec-D). Some nighttime formulas pair an antihistamine with a pain reliever. Understanding what each ingredient does helps you avoid doubling up. If you’re already taking a standalone antihistamine and then add a combination cold product that also contains one, you’re getting a double dose without realizing it.
Reading the “Drug Facts” panel on any allergy product will tell you exactly which active ingredients are inside, their purpose, and their dose per tablet or spray. The active ingredient is what matters, and the brand name is just packaging around it. Store-brand versions contain identical active ingredients at the same concentrations for a fraction of the price.