Benadryl: Side Effects and Cautions

Benadryl (diphenhydramine) is a well-known antihistamine used to treat allergy symptoms, including itchy and watery eyes, sneezing, and runny nose. It may also be used to treat colds, dry coughs, insomnia, and motion sickness.

While typically mild, the side effects of Benadryl may be more intense for some people. Benadryl can be dehydrating, and is known to cause side effects like dry mouth and eyes. It can also cause undesirable and possibly even unsafe side effects like drowsiness and blurred vision.

This article details the side effects of Benadryl. It also lists drugs that may interact with Benadryl, along with who Benadryl is—and is not—recommended for.

Common Side Effects

Sold over-the-counter without a prescription since 1946, Benadryl now comes as a tablet, rapid-dissolving tablet or strip, capsule, liquid capsule, powder, and liquid. A short-acting antihistamine, Benadryl lasts four to six hours.

Although is it generally considered an effective medication, it is not without side effects.

Common side effects of Benadryl include:

• Dry mouth or nose
• Drowsiness
• Headache
• Blurry vision
• Low blood pressure
• Constipation

These side effects should be temporary. There may also be other side effects that are not listed here. If the side effects are ongoing or bother you, contact your healthcare provider.

Serious Side Effects

Benadryl can impair mental and motor functioning, as well as your performance at work or school. It can also impair a person’s ability to operate a motor vehicle or other potentially dangerous equipment.

Additionally, taking more than the recommended amount of diphenhydramine can cause overdose. A person that has overdosed may experience some of the more common side effects of Benadryl, as well as:

• Inability to urinate
• Enlarged pupils
• Ringing in the ears
• Rapid heartbeat
• Agitation
• Confusion
• Seizures
• Delirium
• Depression
• Hallucinations
• Increased sleepiness
• Nervousness
• Shaking or trembling
• Unsteadiness
• Nausea or vomiting
• Seizures

The presence of any of these symptoms should be treated as an immediate medical emergency. Call 911 or get to the nearest emergency room if you suspect an overdose has occurred.

Interactions

Benadryl may interact with medications used for treating health conditions including:

• Medications with anticholinergic effects (meaning that they block the effects of a neurotransmitter called acetylcholine), such as Paxil (paroxetine) and Desyrel (trazodone)
• Medications with CNS depressing effects, such as Xanax (alprazolam) and Klonopin (clonazepam)
• Opioid pain reliever hydrocodone

Do not mix alcohol with Benadryl as it may increase side effects and add to the drowsiness caused by this medication.

Special Considerations

Use of Benadryl in particular individuals can result in amplified side effects, an increased risk of overdose, and other concerns. It's imperative that you check with a healthcare provider before using this medication if you (or someone you care for) are in any of the following groups.

People With Certain Health Conditions

Benadryl works to relieve congestion by drying out nasal passages. While this may be beneficial if you have allergies, the medication can also dehydrate the entire body. This can alter your heart rate, blood pressure, respiration, and ability to eliminate.

Given this, people with certain health conditions should not use Benadryl unless under the supervision of a practitioner. The antihistamine may exacerbate underlying health concerns, including: 

• Closed-angle glaucoma
• Constipation
• Urinary retention
• Enlarged prostate
• Asthma, emphysema, or other lung diseases
• Severe liver disease 
• Heart disease
• High blood pressure
• Seizures
• Thyroid disease

Your healthcare provider can best guide you in determining if Benadryl is safe for you.

Children

Benadryl and other first-generation antihistamines should be used with caution in children and preferably under the direction of a healthcare provider. Benadryl should never be given to a child in order to make them sleepy.

Children may experience similar side effects as adults, or potentially experience unexpected side effects such as excitement, irritability, and insomnia.

The potential for overdose is a major concern in young children. According to a study from 2020, the most common signs of diphenhydramine overdose in children include:

• Rapid heartbeat (tachycardia)
• Hallucinations
• Sleepiness
• Agitation
• Enlarged pupils

Unless promptly treated, diphenhydramine overdose can lead to seizures, coma, and death.

Older Adults

Benadryl is not typically recommended for adults ages 65 and older, except to treat an acute allergic reaction. Older adults may be more affected by side effects than younger adults.

Common side effects such as sleepiness, dizziness, and decreased reaction times may be more pronounced in older adults, especially if they have other medical conditions, and may result in falls or accidents. 

Because of Benadryl's dehydrating effects, it may cause dry mouth, constipation, blurred vision, and urinary retention, which can increase the risk of developing urinary tract infections (UTIs) or affect kidney function.

In addition, diphenhydramine has anticholinergic effects. As a result, anticholinergic drugs can cause temporary cognitive impairment or confusion. Also, there is growing evidence that chronic use of these medications may be associated with the long-term development of dementia.

Pregnant and Nursing Women

Benadryl is a commonly used antihistamine during pregnancy. That said, regardless of how often one plans to take it, the drug should only be used under the direction of a healthcare provider.

Large doses or overdoses of Benadryl can result in premature contractions during pregnancy. Caution should be used when giving Benadryl near delivery, as the side effects in a newborn can include excitement, irritability, and seizures.

Like many other medications, Benadryl is secreted in breast milk and may be passed to a nursing child, causing temporary side effects such as irritability and colic.

Second-generation antihistamines, such as Zyrtec (cetirizine) and Claritin (loratadine), have fewer side effects, are more effective, and are safer medications for pregnant women and those with nursing children.

Alternative Medications to Benadryl

Benadryl is an effective antihistamine, however, its side effects make it less desirable or contraindicated for some.

Newer antihistamines do not typically cause drowsiness and are generally preferred for long-term use over Benadryl. These drugs include:

• Xyzal (levocetirizine)
• Zyrtec (cetirizine)
• Allegra (fexofenadine)
• Claritan (loratadine)
• Clarinex (desloratadine)

Talk to your healthcare provider about your allergy symptoms for recommendations on the best antihistamine for you.

Summary

Benadryl (diphenhydramine) is an effective antihistamine medication for treating allergy and cold symptoms. However, Benadryl is dehydrating and commonly causes dry mouth and eyes. Side effects like drowsiness and blurry vision can be unsafe, so you should avoid operating a vehicle for at least six hours or until the effects of Benadryl have completely worn off.

Some drugs, like opioids, should never be combined with diphenhydramine due to the increased risk of overdose. If you experience signs of overdose, such as confusion, delirium, or hallucinations, call 911 or get to the nearest emergency room right away.