Tiring out or drowsiness is one of the major side effects of some antihistamines, especially older or first-generation antihistamines such as diphenhydramine (Benadryl) and doxylamine succinate (Nyquil).
Also, antihistamines have powerful sedating properties, which is why they are found in a variety of over-the-counter sleep aids.
Why do antihistamines make you tired or drowsy?
Before understanding how antihistamines work, it is important to know about histamines, the chemicals that play a crucial role in the body when there is an immune response to allergens.
When you are exposed to an allergen, your immune system detects and tries to combat it, making your white blood cells and tissue cells release histamines. These chemicals induce an inflammatory response, dilating the blood vessel, increasing blood flow and causing swelling of the tissue.
These are actually the effects of a healing process. For instance, when you are fighting a cold or pollen (allergic rhinitis or hay fever), histamines dilate the blood vessels in your upper respiratory tract, causing nasal congestion. In addition, there is increased fluid leakage and mucus production, resulting in a runny nose. Also, your eyes get watery, itchy and red.
An allergy occurs when your immune system thinks an allergen, such as pollen, mold, dust, or pet dander, is actually harmful. Histamines come to the play and cause allergic symptoms such as sneezing, red or itchy eyes, nasal congestion, and a runny nose.
Antihistamines are often prescribed to ease allergy symptoms, which work by blocking the secretion of histamines, preventing the chemicals from functioning.
Older or first-generation antihistamines, such as diphenhydramine and doxylamine succinate, cross the blood-brain barrier and suppress one of the functions of histamines, which include sleep regulation and wakefulness. This kind of disruption caused by antihistamines in the brain causes fatigue and drowsiness. Newer or second-generation antihistamines, such as cetirizine (Zyrtec), loratadine (Claritin) and fexofenadine (Allegra), are known to cause less drowsiness than older antihistamines, probably they do not cross the blood-brain barrier. They are known to work on the peripheral nervous system, which is why they cause less fatigue or drowsiness.