Antihistamines are common over-the-counter and prescription medications that help with allergy symptoms. They can also help manage other conditions, like digestive issues and motion sickness. Because there are so many options to choose from, it’s best to talk to your healthcare provider or pharmacist about the right one for you.
Antihistamines are a class of medication that commonly treat allergy symptoms, like runny nose, sneezing and itchy eyes. They can also treat some digestive conditions.
Most antihistamines are over-the-counter (OTC) medications, but they also come in prescription form. Antihistamines are also common ingredients in the following OTC medications:
Antihistamines work by blocking histamine receptors. “Anti-” means “against.” Histamine is a chemical your immune system releases to send messages between different cells. Histamine has several functions, but it’s mainly known for its role in causing allergic and anaphylactic symptoms. Allergic reactions can happen due to several substances (allergens), including:
Antihistamines can help manage all these allergies.
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Healthcare providers divide antihistamines into two major subtypes based on which histamine receptors they block:
You have H1 receptors throughout your body, including in neurons (brain cells), smooth muscle cells of your airways and blood vessels. Activation of the H1 receptors causes the well-known allergy and anaphylaxis symptoms. So, H1 receptor antihistamines mainly treat allergy symptoms.
The U.S. Food and Drug Administration (FDA) has approved the use of H1 antihistamines to help treat the following conditions:
Healthcare providers may also use H1 antihistamines to treat insomnia. This is an off-label use of the medication.
The H1 antihistamines further break down into two groups:
First-generation antihistamines generally have more side effects than second-generation antihistamines because of how they work. First-generation antihistamines more easily cross your blood-brain barrier into your central nervous system. This causes drowsiness. Second-generation antihistamines are generally safer because they don’t cause drowsiness and interact with fewer medications.
A few examples of first-generation H1 antihistamines include:
A few examples of non-drowsy H1 antihistamines include:
You have H2 receptors mainly in the cells in your stomach that release acid, smooth muscle cells and heart cells. So, H2 antihistamines treat certain gastrointestinal (GI) conditions.
The FDA approves H2 antihistamines to help treat the following conditions:
Healthcare providers may also use H2 antihistamines to treat indigestion. This is an off-label use of the medication.
If you’re taking an antihistamine to help with stomach issues, you’re likely taking an H2 antihistamine. A few examples include:
Antihistamines work by blocking or reducing a chemical called histamine. It has a role in several different bodily processes. Histamine:
Your body releases histamine when it encounters a threat from an allergen, leading to allergy symptoms, like:
By blocking the effects of histamine, antihistamines help prevent or manage these symptoms. Histamine also affects your nervous system and digestive system. So, antihistamines can also help with certain digestive conditions and things like motion sickness and nausea.
Antihistamines don’t always completely control symptoms. This is because other chemicals besides histamine may be partly responsible for the symptoms. Antihistamines only block histamine.
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Antihistamines come in several forms, including:
There are many types of antihistamines, and each one is different. Make sure to carefully read the label and information for directions that come with the medication. Or follow your pharmacist’s or healthcare provider’s instructions.
Before taking an antihistamine, you should know:
In terms of what not to do when taking antihistamines:
Children and people over the age of 65 are more sensitive to the effects of antihistamines. Use caution and talk to a healthcare provider first.
People take antihistamines for several reasons. When you take an antihistamine likely depends on what kind of allergies you have and how severe they are. For example, you may take an antihistamine:
For many people with environmental allergies, their symptoms are the worst in the early morning. Taking an antihistamine at bedtime may help you or your child feel better in the morning.
Again, if you don’t know what’s best for you or have questions, talking to a healthcare provider or pharmacist is always your best bet.
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Each antihistamine has different possible side effects. You should discuss taking an antihistamine with your healthcare provider. Together, you can decide if the potential benefits of an antihistamine outweigh its potential side effects.
Side effects of first-generation H1 (allergy) antihistamines can include:
Side effects of second-generation H1 (allergy) antihistamines can include:
Side effects of H2 (digestive) antihistamines are uncommon but can include:
Antihistamines help millions of people with allergies manage their symptoms. And they’re one of the most commonly used medications. Antihistamines are generally effective, though it may take time to find the one that works best for you.
Even though many antihistamines are available over the counter, they can still cause significant problems if you don’t use them correctly.
The main possible complication is overdose (antihistamine toxicity).
In addition, some antihistamines can make certain health conditions worse. If you have any of the following conditions, you should talk to your healthcare provider first before taking antihistamines:
The toxicity of antihistamines varies greatly depending on the type. But first-generation H1 antihistamines (the ones that make you drowsy) are most likely to lead to antihistamine overdoses.
An overdose can happen if you or your child take too much of an antihistamine. Or it can happen if you take it with other types of medications, like hypnotics or sedatives. These are most often sleeping pills. Children and people over the age of 65 are most at risk for accidental antihistamine overdose.
Symptoms of antihistamine overdose include:
If you or your child has these symptoms, call 911 (or your local emergency number) or get to the emergency room as soon as possible. A severe antihistamine overdose can be fatal if it’s not treated.
To prevent overdoses, always take your medication as prescribed and store antihistamines safely away from children and pets.
It’s safest to talk to your healthcare provider if you’re pregnant, planning to become pregnant or breastfeeding (chestfeeding).
Up to 15% of people use an antihistamine during pregnancy to manage allergic rhinitis or nausea. Many studies show no significant risk of fetal malformations (birth defects) with first-generation antihistamines. Small amounts of antihistamines can pass on to your baby if you breastfeed.
Everyone is different, and researchers are always learning new information. Because of this, your provider will want to talk with you and help you decide what’s best.
Even though you can get most antihistamines at the store, it’s still a good idea to talk to your healthcare provider about taking them. This is especially true if you’re thinking about giving an antihistamine to your child or if you have other health conditions.
Otherwise, call your provider if:
It might help to ask your provider the following questions:
A note from Cleveland Clinic
Histamine tries its best to help and protect you. But it can be oversensitive and overreact, leading to symptoms like rashes, runny nose and watery eyes. That’s where antihistamines can help. If you have allergies, stomach issues or any other conditions and symptoms mentioned in this article, talk to your healthcare provider about your antihistamine options.
Always check with your provider or pharmacist if you have concerns about antihistamines, and always follow the directions on the labels.
Last reviewed on 07/26/2024.
Learn more about the Health Library and our editorial process.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy