diphenhydramine

Diphenhydramine (Benadryl) is an antihistamine used for treating allergic reactions. Histamine is released by the body during several types of allergic reactions and—to a lesser extent—during some viral infections, such as the common cold.

When histamine binds to receptors on cells, it stimulates changes within the cells that lead to the release of chemicals that cause sneezing, itching, and increased mucus production. Antihistamines compete with histamine for cell receptors and bind to the receptors without stimulating the cells.

In addition, they prevent histamine from binding and stimulating the cells. Diphenhydramine also blocks the action of acetylcholine (anticholinergic effect) and is used as a sedative because it causes drowsiness. 

Benadryl (diphenhydramine) side effects include:

• sedation,
• tiredness,
• sleepiness,
• dizziness,
• constipation,
• disturbed coordination,
• drying and thickening of oral and other respiratory secretions, and
• urinary retention.

Diphenhydramine also may cause:

• low blood pressure,
• palpitations (abnormal heartbeats),
• increased heart rate,
• confusion,
• nervousness,
• irritability,
• blurred vision,
• double vision,
• painful urination or difficulty urinating,
• tremor,
• sweating,
• loss of appetite,
• erectile dysfunction, and
• nausea.

Diphenhydramine should be used with caution (if at all) in persons with narrow-angle glaucoma, prostatic hypertrophy (enlarged prostate gland), hyperthyroidism, cardiovascular disease, high blood pressure (hypertension), and asthma.

Diphenhydramine doses vary depending on the formulation and have their maximal effect about one hour after it is taken.

• When used to treat insomnia, it is prescribed at bedtime. Individuals over the age of 60 years are especially sensitive to the sedating and anticholinergic effects of diphenhydramine, and the dose should be reduced.
• A common regimen for treating adult allergic reactions is 25-50 mg every 4-6 hours not to exceed 300 mg daily.
• Insomnia is treated with 50 mg taken 30 minutes before bedtime.

Diphenhydramine adds to (exaggerates) the sedating effects of alcohol and other drugs than can cause sedation.

This includes the following drug classes:

Benzodiazepines for-anxiety

• diazepam (Valium),
• lorazepam (Ativan),
• clonazepam (Klonopin),
• alprazolam (Xanax),

Narcotic pain medications and their derivatives

• oxycodone and acetaminophen (Percocet), 
• hydrocodone and acetaminophen (Vicodin),
• guaifenesin with hydromorphone (Dilaudid),
• codeine,
• propoxyphene (Darvon),

Tricyclic antidepressants

• amitriptyline (Elavil, Endep),
• imipramine (Tofranil),
• desipramine (Norpramin)

Certain antihypertensive medications

• clonidine (Catapres),
• propranolol (Inderal)

Diphenhydramine also can intensify the drying effects of other medications with anticholinergic properties (for example, dicyclomine [Bentyl] and bethanechol [Urecholine], probanthine).

• Diphenhydramine has not been adequately evaluated in pregnant women. It should be used only if the benefit outweighs the potential but unknown risks.
• Diphenhydramine is secreted in breast milk. Because of the risk of stimulation and seizures in infants, especially newborns and premature infants, antihistamines should not be used by nursing mothers.