Some antihistamine medicines available without a prescription can make you drowsy. But they aren't meant for ongoing sleep problems. That condition is called chronic insomnia.

Antihistamines often are used for hay fever and other allergies. They also are in some cold medicines and in sleep medicines that you can buy without a prescription. But not all antihistamines make you sleepy. Those that do are called first-generation antihistamines. First-generation antihistamines that you can get without a prescription usually contain one of the following medicines:

• Diphenhydramine (Benadryl, Advil PM, others).
• Doxylamine (NyQuil Cold & Flu, Nighttime Cold & Flu Relief, others).
• Chlorpheniramine (Chlor-Trimeton, Alka-Seltzer Plus Cold Medicine, others).

Antihistamines that make you sleepy can be useful as a sleep aid once in a while. But tolerance to the effects of these antihistamines develops quickly. That means the more often you take them, the less likely they are to make you sleepy.

Side effects of taking first-generation antihistamines can include:

• Daytime drowsiness.
• Dizziness.
• Dry mouth.
• Ringing in the ears.
• Nausea.
• Abdominal pain.
• Problems with balance and coordination.
• Mucus buildup in the lungs.
• Rapid heart rate, also known as tachycardia.

Be aware, too, that first-generation antihistamines typically are not recommended for adults 65 and older. That's because they have something called anticholinergic properties. Research suggests those properties might raise the risk of dementia. In older adults, these medicines also may raise the risk for the following side effects:

• Confusion.
• Dry mouth.
• Constipation.
• Problems emptying the bladder completely, a condition called urinary retention.
• Blurred vision.

In addition, first-generation antihistamines may cause serious side effects in people who have:

• Narrow-angle glaucoma.
• Conditions that narrow the esophagus, such as a stenosing peptic ulcer.
• Enlarged prostate.
• A condition where the muscles at the narrowest part of the bladder don't open correctly. This is called bladder-neck obstruction.
• Asthma or other breathing problems.
• Liver disease.

If you have any of these conditions, talk to a member of your health care team before you take medicine that includes a first-generation antihistamine.

Antihistamines may be helpful to ease sleeplessness once in a while. But don't rely on them to treat chronic insomnia. If you have sleep problems that last, talk to your health care professional. Together, you can explore better ways for you to get a good night's rest.