Sedating.You may have taken antihistamines in the past and found it hard to concentrate or stay awake. For many people, sedating antihistamines, also called old, classic, or first-generation antihistamines, cause sleepiness, grogginess, and slow reaction time. They may interfere with coordination and cloud your concentration.

Many sedating antihistamines do not require a doctor's prescription. Sold over the counter (OTC), they are less expensive than prescription nonsedating antihistamines.

Nonsedating. Nonsedating antihistamines, also called new, or second-generation antihistamines, are just as effective against nasal allergy symptoms as older medications. They simply do the job with fewer side effects. They make you less sleepy and groggy, and are less likely to cause problems with increased eye pressure, which may worsen glaucoma symptoms.

Another advantage of the newer antihistamines is that they're available in time-release versions. That means you can control your symptoms with only 1 or 2 doses each day compared with older medications, which usually require doses every 4 to 6 hours to maintain their effectiveness.

The newer antihistamines are available only by prescription. Because some can cause serious side effects or interact with other medications you are taking, be sure to let your doctor know all the medications you take. Your doctor can guide you to the right antihistamine for you.