8b85b910_d_s_Flow___Scientific_American__2
[0] Antihistamines [1] Benadryl, Claritin, Allegra [3] In the brain, histamines are important in regulating the sleep-wake cycle. They have also been associated with learning, memory and attention. Older antihistamines can suppress brain activity, leading to drowsiness and learning problems. Newer antihistamines, such as Zyrtec, work more selectively and have fewer side effects. [2] 
During an allergic reaction, histamines bind to receptors along the blood vessels, making them expand and become more permeable so white blood cells can attack the pathogen. Antihistamines block this mechanism, in effect narrowing the blood vessels.
Common Prescription Drugs Alter Blood’s Flow - Scientific American
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