Diphenhydramine belongs to the antihistamine, H1 antagonist class. The medication is chiefly indicated for the treatment of anaphylaxis, allergic reactions, urticaria, sedation, motion sickness, vertigo, nausea and vomiting. Diphenhydramine is effective to control muscle tremors caused by certain toxins and medications. The medication is contraindicated for hypersensitivity, acute asthma attack and lower respiratory tract disease. The active ingredient in the medication, diphenhydramine hydrochloride, works by blocking the influence of histamine at H1 receptor sites. This in turn results in the reduction of smooth muscle contraction.