Drowsiness, Dry Mouth, and More: 7 Antihistamine Side Effects to Know About

GoodRx Health

Many people know that antihistamines are a go-to treatment for allergy symptoms. But they have many other uses. These include motion sickness, acid reflux, and occasional sleeping troubles.

There are two groups of antihistamines: H1 antihistamines (commonly used for allergies) and H2 antihistamines (commonly used for heartburn). H1 antihistamines can be split further into three generations. Older, first-generation antihistamines are more likely to cause side effects. Newer second- and third-generation antihistamines tend to be better tolerated.

But side effects are possible with all antihistamines. It's good to know what to expect when you first start taking them.

Antihistamine side effects at a glance

Most antihistamine side effects are generally mild and can be managed at home. But more serious side effects are possible. First-generation H1 antihistamines are more likely to cause side effects than newer generations or H2 antihistamines.

Common or mild antihistamine side effects include:

• Drowsiness

• Dizziness

• Headache

• Dry mouth

• Constipation

• Diarrhea

• Fatigue

• Blurry vision

• Trouble balancing

• Tinnitus (ringing in the ear)

Rare but serious antihistamine side effects include:

• Higher risk of falls in older adults

• Worsening of certain types of glaucoma

• Trouble urinating

• Confusion

• Abnormal heart rhythms (seen only with certain antihistamines)

• Hyperactivity in children (usually seen only with first-generation H1 antihistamines)

Below, you'll find seven antihistamine side effects and tips for how to manage them.

1. Drowsiness

Drowsiness can happen with any antihistamine. But it's most common with first-generation H1 antihistamines. In fact, some people use this side effect to their advantage if they're having trouble sleeping. Diphenhydramine (Benadryl, ZzzQuil) and doxylamine (Unisom) are sold as over-the-counter (OTC) sleep aids.

Most second- and third-generation H1 antihistamines are less likely to cause drowsiness. They're often marketed as nondrowsy antihistamines. Of them, fexofenadine (Allegra) is the least drowsy option. It's not common for H2 antihistamines, such as famotidine (Pepcid), to cause drowsiness.

If your antihistamine is making you sleepy, try taking it at bedtime. This can help reduce daytime drowsiness. You should also avoid activities that require you to be alert, like driving, until you know how it makes you feel. And avoid combining antihistamines with other medications or substances that can worsen drowsiness. These include opioids, benzodiazepines, and alcohol.

2. Dizziness

Even though some antihistamines can treat motion sickness, they can still cause dizziness. Similar to drowsiness, this side effect most frequently happens with first-generation H1 antihistamines.

Antihistamines have extra risks for older adults. They can raise your risk of falls. Drowsiness and dizziness both contribute to this. First-generation H1 antihistamines have the greatest risk, but all antihistamines have some risk. It's a good idea to ask a healthcare professional if antihistamines are safe for you before taking one.

If you feel dizzy after taking an antihistamine, sit or lie down. And take your time when moving from one position to another (like going from sitting to standing). This can help prevent falls caused by losing your balance.

3. Headache

Headaches are a possible side effect of all antihistamines. Typically, headaches are mild and more likely to happen with first-generation H1 antihistamines.

If this antihistamine side effect happens to you, a few at-home tips can help:

• Stay well hydrated.

• Avoid alcohol.

• Try adding exercise or more movement to your day.

• Optimize your sleep routine.

• Lower your stress levels through mindfulness, calming activities, or breathing exercises.

Ask a healthcare professional about the occasional use of OTC pain relievers. Options include ibuprofen (Advil, Motrin) and acetaminophen (Tylenol). These can help relieve headaches and don't usually interact with antihistamines.

4. Dry mouth

Dry mouth is a common side effect of H1 antihistamines, especially first-generation ones. This happens because they block acetylcholine. This is a chemical in your body that does many things, including making saliva. Medications that block acetylcholine are sometimes called anticholinergic medications.

Dry mouth from antihistamines is typically mild. It goes away if you stop taking the medication. But while you're having symptoms, it helps to stay hydrated. You can also try sucking on ice chips and chewing sugarless gum.

If you need to take antihistamines long term, tell a healthcare professional if dry mouth is an issue. Chronic dry mouth can lead to dental problems over time. A healthcare professional can provide more long-term management tips for dry mouth.

5. Constipation

Constipation is another possible antihistamine side effect. Like dry mouth, constipation happens because these medications block acetylcholine. It can happen with all types of antihistamines. Other mild digestive side effects, such as upset stomach or nausea, may also happen.

You can often manage constipation by drinking more fluids and gradually increasing the amount of fiber in your diet. Regular exercise may also help get things moving again. If these tips don't work, discuss OTC constipation medications with a healthcare professional. These include stool softeners and laxatives.

6. Blurred vision

Blurred vision is another possible antihistamine side effect. It's more common with H1 antihistamines, particularly first-generation ones. And it happens due to antihistamines' effects on acetylcholine. In most cases, blurry vision is mild and resolves if you stop the medication. If it continues after that point, check in with an eye specialist.

More rarely, antihistamines can cause acute angle-closure glaucoma, a serious side effect. Symptoms include blurry vision, severe eye pain, and seeing halos around bright lights. Contact your eye specialist immediately if you have symptoms of acute angle-closure glaucoma. It can cause permanent vision loss if you don't get treatment right away.

People with certain types of glaucoma have a greater risk of this serious antihistamine side effect. So if you're living with glaucoma, ask your eye care team if antihistamines are OK for you.

7. Trouble urinating

Trouble urinating (urinary retention) can be a serious antihistamine side effect. It's more likely to happen with first-generation H1 antihistamines. Like some other side effects, trouble peeing happens when acetylcholine is blocked.

If urinary retention happens, you may feel like you can't empty your bladder. You may also have lower stomach pain or swelling. This antihistamine side effect is rare but can become serious. If you have an enlarged prostate, you may have a higher risk of urinary retention with antihistamines.

Urinary retention can lead to bladder or kidney infections if left untreated. See a healthcare professional as soon as possible if you're having symptoms of it.

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