Diphenhydramine Side Effects

Diphenhydramine hydrochloride (or just diphenhydramine or DPH) is a antihistamine used to treat allergies. This means that it can be used in order to prevent the production of histamine, which is an immune response from the body that causes many of the symptoms of allergies.

For instance if you suffer from hay fever then it will not be the particles themselves that are damaging your system, rather the immune system in your body will be mistaking them for toxins and thus triggering an immune response by releasing histamine and this is what causes your sinuses to become blocked and your eyes to run. Antihistamines such as diphenhydramine then can be used in order to counter this effect and thus prevent the allergic reaction or at least reduce it.

Like many antihistamines, diphenhydramine also has a hypnotic effect and can be used as a sleep aid as a result or as a mild anxiolytic (drug used to treat anxiety). It can additionally be used as an antiemetic in order to prevent nausea and vomiting. Finally it can be used medically as a local anesthetic.

There are however a fair amount of adverse effects connected to the use of diphenhydramine and being fully informed of these can help you to decide if you wish to use it. As of 2002 the FDA in the US has required special labeling to warn against products which include diphenhydramine in their ingredients.

Adverse Side Effects

Tiredness: While diphenhydramine can be useful in treating anxiety, this hypnotic effect will not always be desirable and it can result in strong sedation when you just want to treat allergies. This can leave you feeling less energetic of course and groggy which can prevent you from getting the full enjoyment out of the day’s activities, or focusing properly during work.

However interestingly it is shown that in some rare cases it might cause excitation rather than sedation and particularly among children. Likewise those who use it regularly have been shown to develop a tolerance against the sedative effects and at this point they are mostly placebo. It is recommended that the elderly avoid diphenhydramine.

Anticholinergic: Anticholinergic agents is any substance that blocks the neurotransmitter acetylcholine in the nervous systems. The anticholinergic properties of diphenhydramine means that it also causes dry mouth and throat, an increase in heart rate, pupil dilation, constipation, urinary retention and in some cases of overdose hallucinations and delirium.

Hives: This adverse effect is a result of an allergic reaction (somewhat ironically) that some individuals will experience so it is not commonly expected.

Other side effects you may experience can include:

• Photophobia (sensitivity to light)

• Blurred vision

• Ataxia (motor impairment)

• Flushed skin

• Lack of concentration

• Memory loss

• Irregular breathing

• Dizziness

• Irritability

• Itching

• Confusion

• Decrease in body temperature (particularly in the extremities)

• Erectile dysfunction

• Twitching (usually as the drowsiness begins to cease)

Recreational Use and Overdose

Due to the ability of diphenhydramine to cause hallucinations and delirium it can be used as a recreational drug and in Zambia it is a prohibited substance as a result. It can also act as a ‘potentiator’ meaning that it increases the impact of other drugs – notably opiates and alcohol.

Recreational use without careful measurements is dangerous and overdose can result in coma, cardiac arrest and death. In the case of diphenhydramine poisoning, physostigmine can be given intravenously as an antidote.



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