diphenhydramine
Meaning of diphenhydramine: One drug with several jobs: the drowsiness that limits its daytime use for allergy is the same effect sold as a sleep aid.
Definition of diphenhydramine
Diphenhydramine is a first-generation antihistamine sold without prescription and used to relieve itching, sneezing, a runny nose and other symptoms of an allergic reaction. It works by blocking histamine, the chemical released by certain immune cells during allergy, at one of its receptors. Because it crosses readily into the brain, it also causes marked drowsiness.
What is it used for?
| Use | What it addresses |
|---|---|
| Allergy | Sneezing, runny nose, watery eyes, hives, and reactions to stings |
| Itchy skin | Insect bites, sunburn, and rashes from poison ivy or poison oak |
| Cough and cold | Drying secretions and quieting cough, usually inside combination products |
| Sleep | The sedating effect used deliberately for short-term difficulty sleeping |
| Motion sickness and movement reactions | Its separate blocking of acetylcholine signaling in the inner ear and brain |
The last row explains a use the label alone makes puzzling. Diphenhydramine blocks acetylcholine as well as histamine, and that second action is why it is given for motion sickness and for certain drug-induced movement reactions, and why it was once used for mild tremor in Parkinson disease.
Why does it cause so much drowsiness?
Histamine is not only an allergy signal. In the brain it is one of the chemicals that maintains wakefulness. Diphenhydramine is small and fat-soluble enough to pass out of the bloodstream into brain tissue, where it blocks the same receptor it blocks in the nose and skin, and alertness falls as a result. Newer antihistamine medicines such as loratadine, cetirizine and fexofenadine were designed specifically not to cross, which is why they relieve allergy symptoms with far less sedation. The sedation from diphenhydramine also outlasts the feeling of it, so grogginess and slowed reactions can persist the following morning.
What else does it do?
Blocking acetylcholine produces a recognizable cluster of effects: dry mouth, blurred vision, constipation, difficulty passing urine and a faster heartbeat. Confusion, agitation and falls are more likely in older adults, in whom the drug is generally regarded as a poor choice, and it is one of the medicines most often flagged on lists of drugs to avoid in later life. Tolerance to the sedative effect builds within days, so it loses sleep-promoting value with continued nightly use. Being over-the-counter makes it easy to take unknowingly alongside a combination cold or sleep product containing the same ingredient.
Also known as
Used in a sentence
Diphenhydramine was recommended for the patient's itching related to the allergic skin reaction.
Good to know
Diphenhydramine's drowsiness, a side effect when used for allergies, is the same property that leads to its separate use as a sleep aid, so the two uses share one medication rather than reflecting two different drugs.