Written by Ashima Seth
If you have ever suffered from an allergy and have, consequently, taken medication for it, you are probably acquainted with the term ‘antihistamine’. It’s usually the primary medication prescribed by physicians to deal with allergy symptoms.
However, not many know how antihistamines work, or why they are effective. When your body comes into contact with an allergy trigger, such as pollen or dust, it produces chemicals referred to as histamines. Production of histamines results in the nasal tissue to swell and become stuffy, eyes and nose to run, and itching in nose, eyes and mouth. At times, this may also result in hives, an itchy rash on the skin. Antihistamines reduce or block histamines, thus stopping allergy symptoms. 
First generation antihistamines (Benadryl, for example) have sedative side effects, which is why they are often not prescribed for allergic reactions. Second generation antihistamines, such as Claritin (Loratidine), by contrast, have little or no sedative effects, and have widely come to be used solely for allergic reactions.
However, the sedative and antiemetic, nausea and vomit-preventing, side-effects of first generation antihistamines have been found as a valuable treatment for motion sickness. Promethazine (Phenergan), for example, is a drug to combat nausea which is commonly utilised for motion sickness and causes the heavy sleepiness. 
Motion sickness is caused by conflicting sensory information from organs of the body that aid with perception and balance, including the eyes and the cochlea, or the inner ear. This conflicting sensory information is mostly due to movement such as caused by using transportation like a car, boat or airplane. The symptoms of motion sickness follow a general order: a feeling of uneasiness, followed by the onset of nausea which may come on quite suddenly, but doesn’t necessarily result in vomiting. Other symptoms include paleness of face, abdominal discomfort, dizziness and a breakout of cold sweat.
While antihistamines have proven themselves useful in more than one medicinal field, they must be used with caution. Because they can cause some amount of drowsiness, it is critical to avoid alcohol. You can’t perform any acts that requires superior sensory-motor coordination, such as driving or operating machinery until it is known what the effects of these medications are. People who have issues relating to the central nervous system or who are taking medicines that have an effect on the central nervous system should not be taking antihistamines without consulting their doctor. 
1. “Do I Need Antihistamines for Allergies?” WebMD. WebMD, n.d. Web. 05 May 2017. <http://www.webmd.com/allergies/antihistamines-for-allergies>.
2. “Antihistamines.” Gale Encyclopedia of Medicine, 3rd Ed. Encyclopedia.com, n.d. Web. 05 May 2017. <http://www.encyclopedia.com/science-and-technology/biochemistry/biochemistry/antihistamine>.
3. “Motion Sickness - Dr. Weil's Condition Care Guide.” DrWeil.com, 2 Dec. 2016, <www.drweil.com/health-wellness/body-mind-spirit/ears-nose-throat/motion-sickness/>. Accessed 20 May 2017.