Antihistamines: What You Need To Know Before You Take These

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What Are Antihistamines?

Treating allergies can require various medications and treatments, but usually the very first things to use are antihistamines. You probably have one or two in your medicine cabinet right now.

Let’s take a closer look at how they work, the types available and side effects to be aware of…



How Do Antihistamines Treat Allergies?

Histamine is a natural compound, released by the immune system when you are exposed to an allergen (something you are allergic to).  It increases the permeability of the capillaries (the smallest type of blood vessels) to allow white blood cells and proteins into the affected tissues and sets off the pain and itching we feel.  In the lungs it can cause bronchoconstriction and bronchial smooth muscle contraction, both of which decrease airway diameter, making it harder to breathe.  In our skin, histamine creates the reaction that we call hives or can lead to the development of a rash.  It gives us all the symptoms of allergies—watery, itchy eyes and a congested or runny nose.  Sneezing is due to histamine-associated nerve stimulation.  Histamine also triggers an inflammatory response in many tissues.

So now you can see why histamines can be such a big issue.  Antihistamines, by definition, reduce or completely block histamines from creating problems.  This means they can stop allergy symptoms, or at least decrease them significantly.

Some symptoms like watery, itchy eyes may disappear completely while nasal congestion may need further treatment with a decongestant.


What types of antihistamines are available?

Quite a list of antihistamines are available, some over-the-counter and others by prescription.  Most are an oral formulation such as a capsule, tablet, or liquid.  Others come as nasal sprays or eye drops.



You may already have a good idea of what is available over-the-counter (OTC), and may even have some of these at home.  Diphenhydramine (Benadryl) is probably the most well-known, and is the oldest of this drug still available.  Brompheniramine, clemastine, and chlorpheniramine were some of the first of these ‘allergy drugs’ switched from prescription to over-the-counter.  They are called first generation antihistamines, as a way of separating them from the newer ones.  The side effects are slightly more prevalent for first generation types.  Newer (second generation) antihistamines include fexofenadine (Allegra), loratadine (Claritin), and cetirizine (Zyrtec).

Ketotifen or the combination of naphazoline and pheniramine are used in over-the-counter eye drops.



When over-the-counter antihistamines are not strong enough to give you the relief you need talk with your doctor.  There are multiple prescription antihistamines available:  Azelastine, hydroxyzine, desloratadine, cyproheptadine, carbinoxamine, and levocetirizine.  They are all second generation types.

Emedastine, azelastine, olopatadine, epinastine, and levocabastine are found in prescription antihistamine eye drops.

Azelastine, olopatadine, and ciclesonide are available as nasal spray formulations, by prescription.

Antihistamines are also available in creams that can be used on the itchy skin of eczema.  Because itching usually gets worse towards the evening hours sedating antihistamines tend to be used.  They include promethazine, hydroxyzine, or alimemazine.  The newer antihistamines such as cetirizine or loratadine have not been found to control itchy skin.


Some people prefer to use natural remedies for their allergies.  Butterbur, an herb, contains the active ingredient petasin.  It works like an antihistamine but with fewer side effects.  Vitamin C can help relieve inflammation.  Garlic can also have histamine reducing effects in the body.  Quercetin is a natural antihistamine found in citrus fruits, onions, apples, parsley, tea, tomatoes, broccoli, lettuce, and berries.  Omega-3 fatty acids can help with inflammatory symptoms.

Natural compound nasal sprays are also available.  Each one contains a mixture of different ingredients.


Other medications to help relieve the symptoms of histamine release

When it comes to nasal allergies, many physicians are turning to nasal steroids.  They have been shown to be more effective in relieving allergy symptoms than nasal antihistamines.using-nasal-spray

Nasal allergy symptoms can include quite a congested feeling nose.  The nasal passages swell with fluids in response to histamines.  Antihistamines can help but many times you also need to take a decongestant.  Currently there are two decongestants available as pills and two as nasal sprays.   The pill decongestants are related—phenylephrine and pseudoephedrine (eg Sudafed) —but if one doesn’t work well for you, you should try the other.

Combination medications containing both an antihistamine and a decongestant are widely available.  Oxymetazoline and phenylephrine are available in nasal sprays.

Nasal drainage can lead to a cough.  When the cough gets to be too bothersome you may need to turn to anti-tussives (cough relieving drugs).  There are a variety available over-the-counter.  Dextromethorphan is popular and found in many different brands.  Benzonatate and carbetapentane are also available.


Side effects of antihistamines

Antihistamines can cause troubling side effects.  Drowsiness is the number one problem.  Luckily the newer, second generation types tend to cause less drowsiness.

Talk to your doctor or pharmacist about potential side effects

Talk to your doctor or pharmacist about potential side effects

If one antihistamine causes drowsiness try a different one.  They are all different in the amount of side effects they cause.  Don’t assume you can’t take antihistamines because of side effects until you have tried them all.

Other side effects from antihistamines include:

  •             Dry mouth
  •             Dizziness
  •             Nausea and vomiting
  •             Restlessness or moodiness
  •             Trouble urinating
  •             Blurred vision
  •             Confusion


Nasal sprays act locally, so if side effects are a major problem for you, you might find that using nasal spray antihistamines will give you the relief without the hassle.  They can irritate the lining of your nose and be drying enough to cause nasal bleeding.  Nasal sprays can only be used for about 3 days because longer use will cause rebound congestion when you stop using them.

Talk with your health care provider before taking antihistamines if you have:

  • an enlarged prostate,
  • heart disease,
  • high blood pressure,
  • thyroid problems,
  • kidney or liver disease,
  • or glaucoma.

Also check with them if you are pregnant or nursing as certain antihistamines appear to be safer than others in pregnancy.

Read more on the potential side effects here.


Now that you have a better understanding of antihistamines you will be a better consumer and patient.  You should always know about any medication you are taking.

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