Antihistamines are medicines used to treat allergy symptoms. The body produces a chemical in the body called histamine when exposed to allergens. Histamines are released to attack allergens but they cause symptoms associated with allergies such as itching, sneezing, runny nose, hives, and itchy and watery eyes.
Antihistamines may also be used to treat other conditions such as:
- Motion sickness
- Trouble sleeping (insomnia)
- Anxiety
- Spinning sensation (vertigo)
The main side effect of first-generation antihistamines is drowsiness or sleepiness. This can also occur to a lesser extent with second-generation antihistamines
Other common side effects of antihistamines include:
What Are Types of Antihistamines?
There are two types of antihistamines:
- First-generation antihistamines
- The first type of antihistamines developed
- Usually cheaper and widely available
- They work in the part of the brain that controls nausea and vomiting and so they are also used to prevent motion sickness
- Because first-generation antihistamines commonly cause drowsiness they are sometimes used to treat insomnia
- Examples of first-generation antihistamines include:
- Brompheniramine (Children’s Dimetapp Cold)
- Carbinoxamine (RyVent, Karbinal ER)
- Chlorpheniramine (Chlor-Trimeton, Actifed Cold)
- Clemastine (Dayhist, Dayhist Allergy)
- Dexchlorpheniramine
- Dimenhydrinate (Dramamine)
- Diphenhydramine (Benadryl, Nytol, Sominex)
- Doxylamine (Vicks NyQuil, Tylenol Cold and Cough Nighttime)
- Hydroxyzine (Vistaril)
- Meclizine (Bonine, Medi-Meclizine, Travel-Ease)
- Promethazine (Phenadoz, Promethegan, Phenergan)
- Triprolidine
- Second-generation antihistamines
- These are newer types of antihistamines
- They are used to treat allergy symptoms without causing drowsiness
- Examples of second-generation antihistamines include:
- Alcaftadine (Lastacaft)
- Azelastine (Optivar)
- Bepotastine (Bepreve)
- Bilastine (Ilaxten)
- Cetirizine (Zyrtec)
- Desloratadine (Clarinex)
- Emedastine (Emadine)
- Epinastine (Elestat)
- Fexofenadine (Allegra)
- Ketotifen (Alaway, Zaditor)
- Levocetirizine (Xyzal)
- Loratadine (Alavert, Claritin)
- Olopatadine (Pazeo, Patanol, Pataday)
- Rupatadine (Rupafin)
Antihistamines are available as pills, liquids, gels, nasal sprays, and eye drops depending on the specific medicine and its use. Many are available over-the-counter (OTC) and some require a doctor’s prescription.
From 
https://familydoctor.org/antihistamines-understanding-your-otc-options/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3650962/
https://www.aocd.org/page/Antihistamines