Antihistamines come in tablet, capsule, liquid, injectable and nasal spray forms. They're available over the counter (OTC) or by prescription. Because antihistamines don't help stuffy noses, they're often sold in combination with decongestants.

When You Take Antihistamines to Treat Allergies

  • Antihistamines work best when taken before you're exposed to allergens.
  • They work by preventing histamines from attaching to certain sites on cells and causing allergic reactions and allergy symptoms.
  • Some over-the-counter antihistamines and some prescription antihistamines can cause drowsiness. Be sure you know when it's safe to use them.
  • Don't drink alcohol while taking antihistamines.
  • Don't take antihistamines without your doctor's approval if you're pregnant, trying to get pregnant, or nursing.

For more than 50 years, antihistamines have been the first type of allergy medication chosen to treat nasal allergies. They are a great way to head off allergy symptoms before they start. They are especially effective against runny noses, sneezing, and itching. However, they do little to relieve nasal congestion.

Many antihistamines are available without a prescription, and some are quite inexpensive. Some can cause drowsiness or sedation, however. There are newer antihistamines available that don't cause as much drowsiness. These include the the prescription drug Allegra (fexofenadine), and Zyrtec (cetirizine) and Claritin (loratadine), which are now available without a prescription.

Sedating versus Nonsedating Antihistamines to Treat Allergies

Sedating Antihistamines. 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 Antihistamines. 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 allergy 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.

Some newer antihistamines are available only by prescription, and some are available over-the-counter. Your doctor can help you make the right choice. 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.