Health Glossary


Allergies can be a real bother, often leaving us sneezing, itching, or experiencing other uncomfortable symptoms. Thankfully, antihistamines come to the rescue by providing relief from these allergic reactions. In this article, we'll delve into the world of antihistamines, exploring their definition, types, and potential side effects.

What are allergies?

Allergies are the body's immune system overreacting to substances known as allergens.

Allergens include things like pollen, pet dander, dust mites, certain foods, insect venom, and many other things. When exposed to allergens, the immune system releases a chemical called histamine.

What is histamine?

Histamine is a naturally occurring chemical in the body that plays a role in various physiological processes.

In the case of allergies, histamine is released as a defense mechanism against perceived threats, such as allergens. It binds to specific receptors in various tissues, triggering a cascade of allergic symptoms.

What substances cause allergies?

Allergens (substances that cause allergies) vary widely from person to person. Some common allergens include:

  • Pollen from trees, grasses, and weeds
  • Dust mites and their waste products
  • Pet dander from cats, dogs, or other animals
  • Mold spores
  • Certain foods such as peanuts, shellfish, or eggs
  • Insect stings or bites
  • Medications like penicillin or aspirin

What allergic symptoms do histamines cause?

Histamines trigger a range of symptoms, including:

  • Sneezing
  • Runny or stuffy nose
  • Itchy, watery eyes
  • Skin rashes or hives
  • Swelling, particularly in the face, lips, or tongue
  • Itchy throat or ears
  • Coughing or wheezing

These symptoms can vary in intensity depending on the individual and the allergen involved.

What are antihistamines?

Antihistamines are medications that counteract the effects of histamine, providing relief from allergy symptoms. They work by blocking the histamine receptors, preventing histamine from binding and causing allergic reactions.

How are antihistamines classified?

Antihistamines can be classified into two generations: first-generation and second-generation.

  1. First-generation antihistamines: These were the earliest types of antihistamines developed. They effectively relieve allergy symptoms but often cause drowsiness and other side effects. Examples include diphenhydramine and chlorpheniramine.
  2. Second-generation antihistamines: These newer antihistamines are designed to be less sedating while still providing allergy relief. They are often preferred for daytime use. Some common second-generation antihistamines include cetirizine, loratadine, and fexofenadine.

What’s the difference between first- and second-generation antihistamines?

First-generation antihistamines have been around for a longer time and are more likely to cause drowsiness. They also have a shorter duration of action, meaning they may need to be taken more frequently throughout the day.

Second-generation antihistamines are generally non-drowsy and have a longer duration of action. This allows for once-daily dosing, providing extended relief from allergy symptoms without causing significant drowsiness.

What are some examples of H-1 first- and second-generation antihistamines and H-2 blockers?

There are many prescription and over-the-counter H-1 antihistamines. If you have allergies, you’re likely taking a H-1 antihistamine. A few examples of first-generation over-the-counter and prescription H-1 blockers include:

  • Brompheniramine (Children’s Dimetapp Cold®).
  • Chlorpheniramine (Chlor-Trimeton®).
  • Clemastine (Dayhist®).
  • Cyproheptadine (Periactin®).
  • Dexchlorpheniramine Dimenhydrinate (Dramamine®).
  • Diphenhydramine (Benadryl®).
  • Doxylamine (Vicks NyQuil®, Tylenol Cold and Couth Nighttime®).
  • Hydroxyzine (Vistaril®).
  • Phenindamine (Nolahist®).

A few examples of second-generation over-the-counter and prescription H-1 blockers include:

  • Azelastine (Astelin®).
  • Loratadine (Claritin®).
  • Cetirizine (Zyrtec®).
  • Desloratadine (Clarinex®).
  • Fexofenadine (Allegra®).

If you’re taking an antihistamine to help with stomach issues, you’re likely taking a H-2 antihistamine. A few examples of H-2 antihistamines include:

  • Cimetidine (Tagamet HB®).
  • Famotidine (Pepcid®).
  • Nizatidine (Axid®).
  • Ranitidine (Zantac®).

What are the side effects of antihistamines?

While antihistamines are generally safe and well-tolerated, they can still have potential side effects. It's essential to be aware of these before using them. Some common side effects include:

  1. Drowsiness: This is more common with first-generation antihistamines, as they can cross the blood-brain barrier and affect the central nervous system.
  2. Dry mouth, nose, or throat: Antihistamines can reduce the production of saliva, leading to dryness in these areas.
  3. Dizziness or coordination problems: Some individuals may experience a feeling of lightheadedness or difficulty with balance.
  4. Blurred vision: Temporary changes in vision can occur due to the anticholinergic effects of antihistamines.
  5. Upset stomach or nausea: Mild gastrointestinal symptoms may occur in some individuals.
  6. Increased heart rate: Antihistamines can have a stimulant effect on the heart, causing a slight increase in heart rate.
  7. Urinary retention: In rare cases, antihistamines can affect the bladder, leading to difficulty urinating.

It's important to note that the severity and occurrence of these side effects can vary among individuals. If you experience any concerning or persistent side effects, it's advisable to consult with a healthcare professional.

Can antihistamines cause fever?

Fever is not one of the side effects of antihistamines.

Can antihistamines cause constipation?

Yes, some antihistamines, such as diphenhydramine, do cause constipation as a side effect.

Can antihistamines cause dizziness?

Yes. Dizziness is a common side effect of some antihistamines.

Can antihistamines cause depression?

One study of 92 people with chronic itchiness saw that patients who took the antihistamines cetirizine and hydroxyzine reported an increase in depression and anxiety. The effects of all antihistamines on mood disorders have yet to be studied.

Can antihistamines cause high blood pressure?

If you’re already taking medication for high blood pressure, combining that with an antihistamine can increase your heart rate and raise your blood pressure. Talk to your healthcare provider about your options.

Can antihistamines cause weight gain?

Antihistamines can cause you to gain weight, yes. One antihistamine, cyproheptadine, is used for that reason. Histamine is known to reduce your appetite, so antihistamines cancel that out.

What antihistamines can you take together?

Antihistamines should not be combined unless directed to do so by your healthcare provider under their guidance and supervision. Antihistamines should be used only as directed or you could experience serious side effects. Read labels very carefully.

What should I do if antihistamines don’t work?

Talk to your regular healthcare provider, your pharmacist or get an allergist to help you find ways to treat your allergies. Some allergies can be treated with decongestants or immunotherapy.

Can I take antihistamines if I’m pregnant or breastfeeding?

It’s safest to talk to your healthcare provider if you are pregnant, planning to become pregnant or are breastfeeding. Animal studies have shown that some antihistamines can cause birth defects. Small amounts of antihistamines pass on to your baby if you breastfeed. For these reasons your healthcare provider will want to talk with you and make careful choices (or different choices) if there is any concern for your or your child’s safety.

Are antihistamines safe for dogs?

Diphenhydramine is a common medication used to treat allergies, hives, food allergies, anxiety and other conditions in dogs. However, you should consult your veterinarian about the use of diphenhydramine in your pet. The dosage in dogs is based on their weight plus your veterinarian will want to examine your dog to be sure an antihistamine is the correct drug for the correct diagnosis. If an antihistamine is needed, your veterinarian will want to prescribe a brand that is specific to animals and at a dosage correct for your pet.

Do antihistamines cause dementia?

Long term use of some antihistamines may increase your risk of dementia. Diphenhydramine (Benadryl®) blocks the effects of a neurotransmitter called acetylcholine. This neurotransmitter is vital for memory and learning. Diphenhydramine increased the risk of dementia by 54% in one 3,000 patient study followed for seven years.

What questions should I ask my healthcare provider about antihistamines?

  • What type of antihistamine would work best for me?
  • How do I proper take the prescribed antihistamine?
  • What side effects might occur with the recommended medication?
  • What antihistamine won’t interfere with the current medications I am taking?
  • When, or for what conditions, does taking an antihistamine that would make me drowsy make sense?
  • Can I live my life normally while using this medication? Can I drive? Can I operate heavy machinery?
  • Can I take antihistamines if I am pregnant, planning to become pregnancy or am breastfeeding?
  • Can antihistamines be safely given to my child?
  • What are the consequences if I don’t take an antihistamine to help with my allergies?

A note from Vitaliboost

Histamine is one piece to a very large puzzle that makes up your body. The chemical does its best to regulate help your heart and lungs and protect your body from foreign allergens, among other roles. But it can be oversensitive, and it can overreact, and that’s where antihistamines can help. If you’re have allergies, stomach symptoms or any of the other conditions and symptoms mentioned in this article, talk to your healthcare provider about your options. Your symptoms may be able to be treated.

Always check with your healthcare provider and your pharmacist if you have concerns about antihistamines, and always follow the directions on the labels!



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