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Why Do Antihistamine Medications Make You Sleepy?

Drowsiness is one of the main side effects of some antihistamines, such as diphenhydramine, which can be found in Benedryl and doxylamine succinate, which is found in Nyquil. And because of their powerful sedating abilities, antihistamines are also the main ingredients that are found in many over-the-counter sleep aids. But why do antihistamine medications make us sleepy? The answer to this question starts with histamines.

What are Histamines?

Histamines are chemical compounds that play a couple of roles in the body. They are best known for being involved in the body’s local immune responses. Histamine is a natural chemical in the body that performs numerous functions such as playing a part in the allergy response.

Allergies occur when your immune system mistakenly thinks a harmless foreign substance, such as pollen or pet dander, is actually dangerous. Histamines will jump to action, which will cause the range of symptoms associated with allergies such as sneezing, itchy eyes, chest congestion, wheezing, and more.

When you get injured or your immune system detects a potentially dangerous foreign substance, certain white blood cells and tissue cells release histamines that pursue and attach to other cells that have a histamine receptor. The histamines induce an inflammatory response, where they will dilate the blood vessels that will increase the blood flow to the site of injury or invasion. There are some side effects that come along with this healing process. An example is when you’re fighting off a cold, histamines will widen the blood vessels in your nasal cavity which will cause nasal congestion.

What are Antihistamines?

Antihistamines are usually used to ease allergy symptoms. They work by blocking histamines attachment to receptors, which prevent the compounds from carrying out their functions. They can cross the blood-brain barrier and inhibit one of the other functions of histamines, which is the role they play in regulating sleep and wakefulness. This disruption of the action of histamines in the brain results in drowsiness.

Older Antihistamines

Because first-generation antihistamines, such as diphenhydramine (Benadryl) or hydroxyzine (Atarax), block the action of histamine at this receptor, they block histamine’s effect on wakefulness. When the histamine is blocked from triggering the receptor and participating in the sleep-wake cycle, the brain sees this as the “sleep mode” of the receptor, and you will feel drowsy. These older antihistamines don’t discriminate between which histamine receptors they block.

Older antihistamines have a sedating effect because the medication penetrates the blood-brain barrier, which means that the medication not only works in treating an allergy but also affects the brain. This results in a feeling of lethargy. In fact, some older antihistamines are prescribed specifically because they help with sleep and relaxation. However, this can be an unwanted and serious side effect for someone who needs to drive or operate heavy machinery.

Newer Antihistamines

Newer antihistamine medications, such as loratadine (Claritin) or desloratadine (Aerius), have different chemical structures that don’t allow them to readily enter the brain. Even though they have action at the H1 receptor, they can’t access the receptors that are present in the brain, which means they do not cause drowsiness. This is also the reason why newer and different antihistamines have less drowsiness effects associated with them. Newer, once-a-day antihistamine medications have no effect on the brain and just function in decreasing allergy symptoms. They block the histamines which cause inflammation, mucous, itching and hives. However, they can still cause drowsiness in a small percent of people who take them. If this is the case for you, you should talk to your doctor about other approaches you may be able to take.

Treatment with Dr. Shukla

Dr. Shukla of Brooklyn New York has been in practice for more than 15 years. He has provided the answers and treatments to many children suffering from asthma, allergies, sleep apnea. Here at the Asthma, Allergy and Sleep Center of New York, we provide you with a personalized, boutique approach to health care and pediatric pulmonology all under one roof. We emphasize prevention and use our advanced diagnostic technologies to give you and your child the best possible care. Whether you need to see a sleep doctor, need allergy treatment or need a pulmonologist, contact Dr. Shukla’s New York offices today for an appointment.

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