Whether your respiratory problem is as minor as a cough or as serious as an asthma attack, the drug that will most likely be prescribed for you will fall under the category of respiratory medications.
Histamine is a body chemical that, when released in the body, typically causes swelling and itching. Antihistamines counteract these allergy symptoms by blocking the effects of histamine and are used for mild respiratory allergies, such as hay fever.
Diphenhydramine and other antihistamines are relatively slow-acting. Severe allergic reactions sometimes require the use of epinephrine (which is not an antihistamine). In its injectable form, epinephrine acts very quickly.
Some types of antihistamines are also used to prevent or treat motion sickness. The antihistamines diphenhydramine, meclizine, and scopolamine, for example, are often used specifically for this purpose.
Antitussive medications control coughs. There are many nonprescription antitussives available; most contain dextromethorphan. Codeine is a narcotic antitussive that is an ingredient in many prescription cough medications. Before they can relieve a cough, these preparations must be absorbed into the blood, circulate through the system, and then act on the brain.
Bronchodilators (agents that open airways in the lungs) and medications that relax smooth-muscle tissue (such as that found in the lungs) are used to improve breathing. Aminophylline and theophylline are oral bronchodilators commonly used to relieve the symptoms of asthma and pulmonary emphysema. Albuterol and metaproterenol are bronchodilators available orally or as inhalants and act directly on the muscles of the bronchi (breathing tubes).
Decongestants constrict blood vessels in the nose and sinuses to open air passages. They are available in different forms: as oral preparations, nose drops, and nasal sprays.
Oral decongestants are slow-acting but do not interfere with the production of mucus or movement of the cilia, hairlike structures in the respiratory tract that help sweep mucus and debris out of the air passages. They can increase blood pressure, however, so they should be used cautiously by patients who have high blood pressure.
Topical decongestants (nose drops or spray) provide almost immediate relief. They do not increase blood pressure as much as oral decongestants, but they do slow the movement of the cilia. People who use these products may develop a tolerance for them. (Tolerance can be described as a need for ever-increasing dosages to achieve a beneficial effect.) One disadvantage of developing a tolerance is that as the dosage increases, the risk of side effects also increases. Topical decongestants should not be used for more than a few days at a time.
Expectorants are used to change a dry, nonproductive cough into a productive one (that is, one that brings up phlegm). Expectorants are supposed to increase the amount of mucus that is produced, but drinking extra water or using a vaporizer or humidifier is probably more effective at increasing mucus production.