8 Benadryl Side Effects You Should Know About

If you’ve scanned the aisles at your local pharmacy for allergy medications, you’ve probably seen Benadryl (diphenhydramine). Benadryl is a commonly purchased antihistamine that’s available over the counter (OTC). Its active ingredient is also found in many OTC sleep aids and cold medications.

While Benadryl is effective at relieving allergy symptoms like itching and a runny nose, it can cause many side effects. Here, we’ll review eight Benadryl side effects you should be aware of.

1. Drowsiness

Drowsiness is one of the most commonly reported Benadryl side effects. Benadryl works by blocking a chemical called histamine from attaching to its receptors (chemical binding sites). When Benadryl slips into the brain and blocks histamine receptors, it can cause sleepiness.

It’s also important to be careful not to combine Benadryl with other medications and substances that can cause drowsiness, like opioids and alcohol. This mixture can result in excessive drowsiness.

There are a few ways to handle drowsiness from Benadryl. Taking the medication close to bedtime might make sense because you’ll already be going to sleep. But if you need to take it during the day, you should avoid activities that require you to be alert.

Driving, operating machinery, and similar tasks should be avoided until Benadryl wears off (about 6 hours after a dose). Switching to a non-drowsy antihistamine, such as Allegra (fexofenadine), is also an option if the drowsiness from Benadryl is too bothersome.

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2. Dizziness

Even though the active ingredient in Benadryl is sometimes included in motion sickness medications, it can actually cause dizziness as a side effect. But other conditions might cause dizziness as well.

If you experience dizziness while taking Benadryl, find a safe place to sit or lay down. This can help prevent falls caused by losing your balance.

Dizziness from Benadryl usually goes away on its own. But if it doesn’t resolve with continued use, it’s best to stop taking Benadryl and ask your pharmacist or healthcare provider for an alternative. If your dizziness doesn’t go away after you stop taking Benadryl, contact your healthcare provider for further evaluation.

3. Blurry vision

Benadryl can also cause blurry vision. Although this side effect is less common, it can raise your risk of falls.

Antihistamines like Benadryl can also cause acute angle-closure glaucoma in rare cases. People with narrow-angle glaucoma are the most at risk for this serious side effect. Symptoms of acute angle-closure glaucoma include blurry vision, severe eye pain, and seeing halos around bright lights.

This condition is serious and can result in permanent vision loss if you don’t get immediate treatment. Contact your healthcare provider right away if you have symptoms of acute angle-closure glaucoma.

Mild episodes of blurry vision should get better on their own. If you continue having blurry vision while taking Benadryl, you may need to stop taking it completely. If your blurry vision continues after that point, ask your healthcare provider about the next steps you should take to resolve your symptoms. In some cases, they may refer you to an eye specialist.

4. Higher risk of falls

As mentioned above, Benadryl can increase the risk of falls, especially in older adults. Other side effects such as drowsiness, dizziness, and blurry vision may all contribute to the higher risk of falling.

Higher fall risk is a common side effect of first-generation antihistamines, like Benadryl, which have more effects on the brain. Second- and third-generation antihistamines like Claritin (loratadine), Zyrtec (cetirizine), and Allegra are often considered safer options. These tend to have fewer side effects and carry a lower risk of falling.

If you're 65 or older, or if you’re prone to falling, check with your pharmacist or healthcare provider before taking Benadryl. They might recommend that you try a different medication that’s safer.

5. Dry mouth

Dry mouth is a common side effect of antihistamines, including Benadryl. It is what is known as an anticholinergic side effect, which occurs when the effects of acetylcholine are blocked in the body. Acetylcholine is a chemical that regulates several body functions, including saliva production. That’s why blocking it can cause dry mouth.

If you experience dry mouth while taking Benadryl, it will usually resolve on its own. But while you’re having symptoms, it helps to stay hydrated. You can also try sucking on ice chips and chewing sugarless gum.

If your dry mouth symptoms don’t improve after a few days, check with your healthcare provider or pharmacist. They may suggest another medication that has a lower chance of causing dry mouth.

6. Constipation

Constipation is another possible Benadryl side effect. Similar to dry mouth, constipation is an anticholinergic side effect. When acetylcholine is blocked, it also slows the movement of food through the intestines, potentially causing constipation.

If you develop constipation after you start taking Benadryl, know that it’s usually temporary. You can help get things moving by drinking more water and increasing your fiber intake. You can also try taking OTC medications for constipation, like laxatives or stool softeners.

Check in with your healthcare provider if your constipation isn’t improving after a few days. If it’s severe or gets worse at any point, contact your care team right away, since severe constipation can lead to more serious bowel problems.

7. Hyperactivity in children

Normally, Benadryl causes people to feel sleepy or drowsy, as discussed earlier. But in some cases, it can have the opposite effect in children and cause hyperactivity. This Benadryl side effect hasn’t been well studied, so there’s not a lot of information on how common it is. Some case reports indicate that the way a child’s body breaks down Benadryl might be related to the medication causing hyperactivity.

There’s not currently a reliable way to predict how your child will respond to Benadryl. This is why the medication isn’t recommended for kids under 6 years old without a provider’s OK.

If your child needs Benadryl, it may be a good idea to give them a first dose at home. If you notice abnormal or unusual behavior in your child after they take Benadryl, check with their healthcare provider.

8. Memory effects

You may have heard that Benadryl can cause dementia, which results in memory loss and other cognitive issues. There are some studies that have found an association between anticholinergic medications, such as Benadryl, and dementia. But the evidence isn’t conclusive. Other studies have also found that antihistamines like Benadryl were not associated with dementia.

Regardless, there are a few things you can do to lower your risk of memory issues that may hypothetically be caused or worsened by Benadryl. Taking the lowest effective dose for the shortest time necessary can lower this risk. Benadryl is not intended to be taken on a daily basis for seasonal allergies or to help with sleep. Discuss better options for daily treatment of these symptoms with your healthcare provider.

When should I contact my healthcare provider about Benadryl side effects?

You should contact your healthcare provider or pharmacist anytime you have questions about Benadryl side effects. It’s best to know beforehand if there’s a reason for you to avoid this medication. For example, if you’re 65 or older, or if you already have a higher risk for falls, Benadryl may not be the safest choice for you.

Most mild Benadryl side effects can be managed at home. But if you have severe side effects, such as eye pain along with blurry vision, excessive drowsiness, or persistent constipation, contact your healthcare provider right away. They will help determine whether you need further medical care.

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Written by:

Austin Ulrich, PharmD, BCACP

Austin Ulrich, PharmD, BCACP, is a board-certified ambulatory care clinical pharmacist. In his early career, he provided direct patient care in a variety of settings, including hospital and community pharmacies, and in a primary care clinic as a clinical pharmacist.

Reviewed by:

Yehun Lee, PharmD Candidate

During her doctoral program, Yehun has worked for Tops Pharmacy for 4 years as a pharmacist intern. There, she has been able to collaborate with other healthcare providers and patients alike.

Reviewed by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

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