Antihistamines commonly relieve symptoms associated with allergies, cold and flu. They also serve a purpose for use in medications that help with itch from rashes and motion sickness. Antihistamines work by blocking the effects of histamine, a neurotransmitter in the body responsible for wakefulness, alertness and reaction times. The American Academy of Family Physicians notes that antihistamines can benefit allergy symptom relief; however, understanding side effects such as drowsiness is important prior to taking a medication of this kind. Consultation with a physician may offer additional solutions for symptoms to reduce the unwanted side effect of drowsiness.
Doxylamine
The antihistamine doxylamine is available over-the-counter (OTC) or in prescription strength as a liquid or in pill form. The U.S. National Library of Medicine notes that doxylamine is primarily used for the short-term treatment of insomnia, or in combination with other medications, it is useful in the treatment of cold symptoms. Doxylamine is not recommended for use longer than seven days. Quick onset of drowsiness is common when using this medication.
Diphenhyrdramine
The antihistamine diphenhydramine has several uses such as relieving symptoms associated with allergies and cold. Additional uses for this antihistamine include treatment for insomnia and abnormal muscle movements associated with early stage Parkinson's disease. Diphenhydramine comes in a rapid disintegrating tablet for quicker relief, as well as pill and liquid form. Diphenhydramine is available over-the-counter and by prescription. The Mayo Clinic cautions that use of this medication may lead to overdose symptoms associated with abdominal pain, and to consult a physician when using this medication in combination with other prescriptions.
Promethazine
Promethazine is used as an antihistamine; however, this medication comes from a group of medications called phenothiazines. Promethazine has sedating effects and the American Cancer Society indicates use of this medication for treating nausea, vomiting and motion sickness associated with cancer symptoms. This antihistamine slows the central nervous system causing drowsiness as a primary means for symptom relief. Promethazine is available by prescription only.
