Diphenhydramine, most commonly known by the trade name Benadryl®, is an antihistamine. Antihistamines combat the effects of histamine released during allergic reactions, which can cause hives, itching, sneezing, and congestion. Diphenhydramine is also used to treat motion sickness, nausea, coughing, and insomnia. Less commonly, it is used to treat the muscle spasms associated with mild forms of Parkinson's disease as well as the abnormal muscle movements caused by psychiatric drugs in the phenothiazine family.
Diphenhydramine is an over-the-counter drug when used in tablet, capsule, or liquid form, but is also prescribed as an injection. Injections are typically only used in medical settings to treat allergic reactions to blood or in lieu of epinephrine. It works as an antihistamine by blocking cell receptors before histamine can bind with a cell. When the antihistamine binds instead, the cell is not stimulated, and hence, the allergic symptoms are not produced. This also succeeds in blocking acetylcholine action, which is not necessarily a desired effect for those taking diphenhydramine for its antihistamine benefits.
Blocking acetylcholine action is called the anticholinergic effect. This can cause constipation, dry mouth, blurry vision, dizziness, and drowsiness. The drowsiness associated with diphenhydramine is, however, the sought out effect of the medication when it is purchased as a sleep aid in over-the-counter drugs such as Unisom® or Nytol®. When used as a sleep aid, it is not recommended for extended use. If sleep issues persist for longer than two weeks, a doctor should be consulted as a tolerance to the sedating effects of diphenhydramine develop quickly.
The full range of side effects associated with diphenhydramine include: drowsiness, dizziness, constipation, blurred vision, coordination issues, drying of respiratory secretions, low blood pressure, heart palpitations, motor impairment, sensitivity to light, difficulty urinating, short term memory loss, difficulty concentrating, decreased appetite, upset stomach, hallucinations, and irritability.
Anyone taking other sedating medications such as anti-anxiety medications, narcotic pain relievers, anti-depressants, sleeping pills, or alcohol should use caution as the drowsiness associated with diphenhydramine will be markedly increased. Anyone over sixty may also notice a stronger sedative effect, so dosages may need to be decreased. No one taking this medication should operate heavy machinery or a motor vehicle until the sedative effects have worn off.
Diphenhydramine is secreted in breast milk, so nursing mothers should not take it. Those with the following medical conditions should consult a doctor before using this medication: asthma, glaucoma, enlarged prostate gland, ulcers, heart disease, high blood pressure, seizures, thyroid problems, or bowel obstructions.