Understanding Medications for Seasonal Allergies

More than 50 million Americans suffer from seasonal allergies and spend billions of dollars on over-the-counter and prescription medications for symptom relief.

You may be curious why you are more susceptible to discomfort from pollen than another. The problem starts with the immune system that is active in your nose1, eyes, and lungs.  This picture helps to explain what is happening in the nose.

When you breathe in pollen from a flower or weed, tiny particles from the pollen activate switches in the body (step 1 in the picture). The medical term for a “switch” is a “receptor.” This is the first step in a system designed to remove harmful substances from your body. The problem arises when the system overreacts. It is this overreaction that results in runny nose, nasal stuffiness, sneezing and itching. After a cell in the nose detects pollen, it turns on the release of several substances included histamine (step 2). Histamine does many things. When it connects with “histamine receptors” in the lining of the nose (step 3), it triggers runny nose and nasal congestion (step 4).  Histamine also activates nerves in the nose.  This results in itching and sneezing (step 5).

Medications such as “antihistamines” help to decrease these symptoms. Antihistamine medications connect to “histamine receptors” and block the effect of histamine.  This effectively turns off the histamine response (step 6).  Antihistamine medications such as Benadryl, Zyrtec and Allegra work in this manner.  

Another type of medication for nasal stuffiness from allergies is a steroid spray. Steroid medications also stick to receptors inside many cells of the body. In cells of the immune system, steroids slow down how well the cells work. In step 2 (see the picture above), immune cells produce histamine and other chemicals. Nasal steroids cause the cells to make less of the chemicals like histamine. The end result is an alleviation of the symptoms – runny nose, nasal congestion, itching and sneezing.

Antihistamines and nasal steroids work very well and have provided millions of people with symptom relief. But like every drug, there are side effects. One important side effect of antihistamines is sleepiness. This bothersome side effect happens because histamine receptors are also in the brain, and they plan an important role in brain activity. The next picture gives you an idea of what happens when an antihistamine medication gets into the brain.

The first step is connection of histamine to histamine receptors in the brain (step 1). One of the effects of this switch is to activate brain responses that maintain alertness and control the sleep-wake cycle (step 2).  When an antihistamine drug gets into the brain, this switch is turned off resulting in sleepiness (step 3). 

It was a major advance when medications like Zyrtec and Allegra were invented. They are less likely to reach the brain. Because of this, they work very well for allergy symptoms in nose, eyes, and skin, but much less commonly cause drowsiness.

Nasal steroids are also less likely to have effects on the brain. Steroids taken by mouth can have affects all over the body and this short blog cannot explain all of the side effects. However, nasal steroids tend to be tolerated very well because most of the medication stays in the nose and works on the immune system in that location. Much less of the steroid gets access to other regions of the body such as the brain.

It is important to understand how these medications work since many can be purchased without a prescription. Unfortunately, there are thousands of products that include medications to control runny nose, itchy eyes, and itchy skin. Many of these products contain antihistamines that do cause drowsiness. You want to be sure about this before you take a new over the counter product.  You would not want to drive or operate machinery at work when taking some of these over the counter medicines. For elderly people, taking these products can put them at risk of a fall and breaking a hip.

Freemedicationreview.org has made a program freely available on the web to let you know when medications you take for allergy symptoms can increase your risk of sleepiness. The use of this program is anonymous and no registration is required.

Click on the link below to start your free medication assessment. This will give you important information for a discussion with your doctor to make sure that all medications taken, including over the counter, do not put you at an increased risk of an accident.


By: Jeffrey Huth, M.D., Ph.D.

July 7, 2019


1.        Wheatley LM, Togias A. Clinical practice. Allergic rhinitis. N Engl J Med [Internet] 2015;372(5):456–63. Available from: http://www.ncbi.nlm.nih.gov/pubmed/25629743

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