What Are The Side Effects Of Benadryl

In our latest question and answer, the pharmacist discusses all of the potential side effects of the antihistamine Benadryl (diphenhydramine).

What Are The Side Effects Of Benadryl
Jul 03, 2018

Simon asked

Can you list all of the potential side effects of Benadryl?


Benadryl PillsBenadryl (diphenhydramine) is a sedating "first-generation" antihistamine that is most commonly used for the treatment of seasonal allergies, allergic reactions and insomnia. 

Side Effects Of Benadryl

Overall, Benadryl is well tolerated and side effects are generally mild in severity. If they do occur, they should subside 4 to 6 hours after dosing, although this can be extended in those with kidney disease and in the elderly. Below, we discuss the most common side effects associated with Benadryl.

Central Nervous System Effects

Benadryl is a central nervous system (CNS) depressant, which can commonly cause:

  • Drowsiness
  • Dizziness
  • Sedation
  • Somnolence

Other less frequently occurring CNS effects include:

  • Weakness
  • Confusion
  • Slurred speech
  • Muscular weakness
  • Fatigue
  • Headache

Rarely, Benadryl can cause paradoxical excitation, but is more likely to occur in children according to the prescribing information. In addition, certain individuals with genetic variations in the CYP2D6 metabolizing enzyme may be at a higher risk of excitation with Benadryl according to studies. Symptoms of Benadryl excitation include:

  • Restlessness
  • Anxiousness
  • Agitation
  • Insomnia
  • Muscle spasms
  • Nervousness
  • Irritability
  • Seizures

Symptoms of overdose of Benadryl often mirror symptoms of excitation and can include insomnia, nervousness or restlessness, irritability, and seizures.

Gastrointestinal System Effects

Benadryl has strong anticholinergic side effects, which include:

  • Dry mouth (xerostomia)
  • Constipation
  • Abdominal pain
  • Diarrhea
  • Abdominal cramping
  • Dry eyes
  • Appetite changes
  • Thickening of bronchial secretions
  • Urinary retention
  • Pupil dilation
  • Blurred vision

Dermatologic Side Effects

Dermatological adverse effects of Benadryl are uncommon, but may include:

  • Rash
  • Ithcing
  • Hives
  • Photosensitivity

In addition, topical application of Benadryl can cause an allergic-type contact dermatitis, although this is thought to be rare.

Cardiovascular Effects

Cardiovascular effects of Benadryl do not typically occur at usual dosages and are more associated with an overdose. Potential symptoms include:

  • Increased heart rate
  • Palpitations
  • Angina (chest pain)
  • Heartbeat outside the normal rhythm
  • Arrhythmias
  • Hypotension and hypertension


As mentioned at the beginning of the article, most side effects of Benadryl are mild to moderate in severity. If they occur, they should subside relatively quickly. More severe side effects, such as cardiovascular reactions, are uncommon and typically occur only when high doses are taken. 

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