Products
Diphenhydramine is commercially available in tablet, drop, and gel forms (e.g., Benocten, Nardyl Sleep, Fenipic plus), among others. It is also known as Benadryl in some countries. Diphenhydramine was developed in the 1940s. It is also a component of the active ingredient dimenhydrinate.
Structure and properties
Diphenhydramine (C17H21NO, Mr = 255.4 g/mol) is present in drugs as diphenhydramine hydrochloride, a white crystalline powder that is very soluble in water. It belongs to the ethanolamine derivatives.
Effects
Diphenhydramine (ATC D04AA32, ATC R06AA02) has antihistamine, antiallergic, depressant, antiemetic, local anesthetic, spasmolytic, and anticholinergic properties. The effects are due to inverse agonism at peripheral and central histamine receptors and antagonism at muscarinic acetylcholine receptors. Furthermore, diphenhydramine also blocks sodium channels, causing local anesthesia. The half-life is in the range of 5 hours. Diphenhydramine also inhibits the reuptake of serotonin. The antidepressant fluoxetine was derived from it.
Indications
Indications include sleep disorders, allergic disorders, itchy skin conditions, motion sickness, dizziness, and colds. Not all drugs are approved for all indications.
Dosage
According to the professional information. As a sleep aid, the drugs are taken in the evening 15 to 30 minutes before bedtime. The duration of treatment should not exceed two weeks without medical supervision.
Contraindications
- Hypersensitivity
- Epilepsy
- Acute bronchial asthma
- Glaucoma
- Prostate enlargement
- Pyloro-duodenal obstruction
- Micturition problems
- Concurrent treatment with an MAO inhibitor, including selegiline.
- Alcohol abuse
- Children (depending on the preparation)
For complete precautions, see the drug label.
Interactions
Other centrally depressant drugs, such as sleeping pills, antidepressants, or neuroleptics, and alcohol may increase central adverse effects. Diphenhydramine should not be taken with MAO inhibitors. Other interactions have been described with anticholinergics and with drugs that prolong the QT interval or cause hypokalemia.
Adverse effects
Possible adverse effects include:
- Fatigue, drowsiness, dizziness, headache.
- Visual disturbances
- Dryness of the mucous membranes of the mouth, nose and throat.
- Gastrointestinal disorders such as nausea, vomiting, constipation, diarrhea.
- Micturition disorders
- Cardiovascular disorders such as changes in blood pressure, cardiac arrhythmias, prolongation of the QT interval.
- Hypersensitivity reactions
Like other sleeping pills, diphenhydramine can lead to dependence. This is especially true with risk factors such as a long duration of therapy, a high dose and a substance dependence in the patient history.