As mentioned above, Antihistamine has two different generation of drugs involved under the different category. Most of the first-generation Antihistamine drugs can be easily bought over the counter (OTC). Some of the first-generation of Antihistamine must go through a doctor’s prescription or pharmacist and cannot be bought easily over the counter (OTC), such example is like Chlorpheniramine. As well as those of the second-generation, Cetirizine such as Zrytec-D and Adezio cannot be bought easily off the counter, it needs a doctor’s prescription or pharmacist approve to get s such drugs. Whereas Zrytec indicated with a R behind it can be bought over the counter and as well as Claritin. If the second-generation of Antihistamine cannot work for the patient, usually doctors will re-prescribe them the first-generation of Antihistamine as it is much stronger. Fexofenadine (Talfast-D) is considered as second-generation of Antihistamine but it has been recently updated on July 1996 therefore it becomes the third-generation of Antihistamine. All the three generations of Antihistamine cause dry mouth, headache, dry nose as its side effects. The side effect depends on individual body level of tolerance for each different drug in the Antihistamine family. Chlorpheniramine is the type of Antihistamine that will cause the user eating it to feel sleepy as it is the first-generation category. Antihistamine is the antidote for Antipsychotic medicines but not to be mixed with alcohol. The first-generation H1 receptor blocker has different interaction. The second-generation Antihistamines can be more relevant in the treatment for allergic conditions. Clinical pharmacology helps practitioner to choose the best H1 Histamines for the patients or users.