Benadryl Side Effects:  Generic name diphenhydramine.  Benadryl is available as a sole ingredient (diphenhydramine) as Benadryl allergy, and combined with multiple other drugs for varying indications.  It is an antihistamine, primarily blocking the H1 histamine receptor, and is used for treatment of allergic reactions.  It is available in 25 mg oral tablets and capsules as well as a 25 mg dissolving strip.    The primary Benadryl side effects are related to Benadryl being a non-specific antihistamine that has significant anticholinergic side effects that cause drowsiness in many if not most people.  This side effect is used to advantage by many people, and diphenhydramine is the active ingredient in many over-the-counter sleep aides including Tylenol PM and others.  The most common Benadryl side effects are sedation and dry mouth.

Black Box Warnings:  The FDA has not placed any black box warnings on Benadryl.

Cautions with Benadryl use:  The sedation side effects are especially of concern in the very young, and in the elderly, and Benadryl is to be used with caution in children under 6, with extreme caution under 2 years old, and with caution in the elderly.  Some individuals are slow hepatic metabolizers using the CYP 2D6 pathway and caution is advised in those individuals.  Caution is also advised with bowel obstruction, with any other drugs that can cause central nervous system depression including alcohol, in patients with significant cardiovascular disease, with chronic obstructive lung disease (COPD, emphysema), asthma, high blood pressure, hyperthyroidism, and enlarged prostate symptoms.

Common Benadryl side effects:  Most of the common Benadryl side effects are related to the sedation and other anticholinergic side effects of Benadryl.  Sedation, drowsiness, incoordination, and central nervous system stimulation (paradoxical) are fairly common.  The whole list of anticholinergic side effects including dizziness, thickened respiratory secretions, constipation, dysuria, dry mouth and other mucous membranes, blurred vision or diplopia and urinary retention are also quite common Benadryl side effects.  Others include hypotension, photosensitivity, excessive sweating, palpitations and tachycardia, headache, and erectile dysfunction.

Serious Benadryl side effects:  Although uncommon there are potential serious Benadryl side effects.  These include anaphylactoid reactions which can seem odd given that Benadryl is commonly used to treat allergic reactions itself.  Other serious Benadryl side effects include various hematologic dyscrasias including hemolytic anemia, agranulocytosis, thrombocytopenia, leukopenia and pancytopenia.  Cardiac arrhythmias, seizures and toxic psychosis are possible with Benadryl.  Heat stroke has  been described with  Benadryl use, likely from reduced sweating although explanation is hypothesis.  Acute labyrynthitis is also possible as a Benadryl side effect.

Benadryl Drug Interactions:  As a drug available over the counter it is especially important for patients to consider potential drug interactions with Benadryl.  The list of possible drug interactions is remarkably long, and for a complete list see the manufacturer’s patient information.  Notable interactions include almost every other drug that can cause sedation including the benzodiazapines, alcohol, tricyclic antidepressants, most pain medications, and many other sedation medications.

Pregnancy and Lactation Class:  Benadryl is pregnancy category B, and its considered probably safe during lactation.

Special Considerations:  The sedating properties of Benadryl make special caution needed for persons driving or operating machinery where falling asleep could be dangerous.  Falls in the elderly may be more common due to hypotension, incoordination or imbalance from Benadryl.  

Unusual Benadryl Side Effects:  The paradoxical CNS stimulation noted by some patients, which seems especially common in children is often unexpected and may seem especially unusual.

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