Antihistamines are medications that block histamine, a chemical your body releases during allergic reactions, and they’re used primarily to treat allergies, hives, and itching. But their uses extend well beyond allergy season. Depending on the type, antihistamines can also help with acid reflux, motion sickness, insomnia, and vertigo.
How Antihistamines Work
When your body encounters something it’s allergic to, like pollen or pet dander, it releases histamine. Histamine latches onto specific receptors on cells throughout your body, triggering the familiar cascade of symptoms: sneezing, itchy eyes, a runny nose, and skin reactions. Antihistamines work by occupying those same receptors first, so histamine can’t attach and set off symptoms. They don’t stop your body from producing histamine. They just prevent it from doing its job.
There are two main types of histamine receptors that matter here. H1 receptors are found in your nose, eyes, skin, and airways, and they drive allergy symptoms. H2 receptors are in your stomach lining and control acid production. Different antihistamines target different receptors, which is why some treat allergies while others treat heartburn.
Allergy and Itch Relief
The most common reason people reach for an antihistamine is allergies. H1 antihistamines treat seasonal allergies (hay fever), year-round allergies to dust mites or pet dander, and allergic skin reactions like hives. They reduce sneezing, runny nose, itchy and watery eyes, and nasal congestion. They’re also widely used for general itching, whether from an allergic reaction, an insect bite, or a skin condition like eczema.
Familiar over-the-counter options include cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra), which are all newer, second-generation antihistamines. Older, first-generation options include diphenhydramine (Benadryl), chlorpheniramine, and clemastine. Prescription options like hydroxyzine are sometimes used for more severe itching or hives that don’t respond to over-the-counter choices. Nasal sprays containing azelastine can target nasal congestion directly, and antihistamine eye drops work specifically for itchy, watery eyes.
Stomach Acid and Heartburn
A separate class of antihistamines, called H2 blockers, targets the histamine receptors in your stomach. After you eat, your body releases histamine that binds to H2 receptors on stomach cells, signaling them to produce acid. H2 blockers prevent that binding, which reduces acid output. Common H2 blockers include famotidine (Pepcid) and cimetidine (Tagamet).
These medications treat conditions driven by excess stomach acid: heartburn, acid reflux, stomach ulcers, and duodenal ulcers. They can relieve indigestion, stomach pain, nausea, and that sour taste from acid creeping up into your throat. For occasional heartburn, an H2 blocker taken before a meal can prevent symptoms before they start. For ulcers, they reduce acid production long enough to let damaged tissue heal.
Motion Sickness and Vertigo
Some first-generation antihistamines have a useful side effect: they suppress signals in the brain that trigger nausea and dizziness. Meclizine is the most commonly used antihistamine for this purpose. It’s taken to prevent and control nausea, vomiting, and dizziness from motion sickness, typically 25 to 50 milligrams about an hour before travel. It’s also prescribed for vertigo caused by inner ear problems, at doses up to 100 milligrams per day. Dimenhydrinate (Dramamine) is another antihistamine used specifically for motion sickness.
Sleep Aids
Many over-the-counter sleep aids use diphenhydramine or doxylamine as their active ingredient. Both are first-generation antihistamines whose drowsiness-inducing effects have been repurposed for short-term insomnia relief. If you’ve ever taken a nighttime cold medicine and felt knocked out, that sedation was likely coming from an antihistamine. These work because first-generation antihistamines cross into the brain easily and block histamine activity there, and histamine plays a role in keeping you awake and alert.
First-Generation vs. Second-Generation
The biggest practical difference between older and newer antihistamines is sedation. First-generation antihistamines (diphenhydramine, chlorpheniramine, hydroxyzine) cross into the brain readily, which makes them effective for sleep and nausea but causes significant drowsiness, impaired concentration, and slower reaction times. These effects make driving or operating machinery risky.
Second-generation antihistamines (cetirizine, loratadine, fexofenadine) were designed to work primarily outside the brain. They relieve allergy symptoms without making most people drowsy, which is why they’re preferred for daytime use. Cetirizine can still cause mild drowsiness in some people, but fexofenadine and loratadine are the least sedating options available.
First-generation antihistamines also have anticholinergic effects, meaning they block another chemical messenger in the nervous system beyond just histamine. This creates additional side effects: dry mouth, dry eyes, blurred vision, constipation, difficulty urinating, and a rapid heartbeat. These effects tend to be more bothersome in older adults.
Risks of Long-Term Use
Short-term or occasional use of antihistamines is generally well tolerated. The concern arises with long-term, daily use of first-generation antihistamines, particularly in older adults. A University of Washington study tracked nearly 3,500 adults aged 65 and older for an average of seven years. Those who used anticholinergic drugs (a category that includes first-generation antihistamines like diphenhydramine) for the equivalent of three years or more had a 54% higher risk of developing dementia compared to those who used them for three months or less. The risk increased with cumulative dose.
This doesn’t prove these drugs cause dementia. It’s an association observed in a long-term study. But acetylcholine, the chemical messenger these drugs block, is directly involved in learning and memory. That biological link gives the finding weight. Second-generation antihistamines have far less anticholinergic activity and are not associated with the same concern, which is another reason they’re generally the better choice for ongoing allergy management.
Mixing Antihistamines With Alcohol
Antihistamines and alcohol both cause drowsiness, and combining them amplifies that effect. Even small amounts of alcohol alongside an antihistamine can impair your coordination and judgment enough to make driving dangerous. The risk is highest with first-generation antihistamines, but it applies to second-generation options as well. Many combination cold and flu products also contain antihistamines, so you may be taking one without realizing it. Checking the active ingredients on any cold, cough, or allergy product before drinking is a simple way to avoid an unexpectedly strong sedative reaction.