Some common antihistamine medications include:

Antihistamines are administered though several types of methods, including:

• Tablet. Solid form of medication that is swallowed. The medication dissolves and is absorbed into the bloodstream through the lining of the stomach and intestines.
  
  
• Liquid. Liquid form of medication that is swallowed. The medication is already dissolved and therefore is absorbed more quickly into the bloodstream through the lining of the stomach and intestines.
  
  
• Nasal spray. Sprays that distribute medicine – through the nose – directly to the nasal passages in the form of a fine mist.
  
  
• Eye drop. A sterile solution or medicine that is applied directly to the surface of the eye in the form of liquid drops.
  
  
• Topical cream. Medications in lotion or ointment form that can be spread directly onto the skin where it is absorbed into the body.

Antihistamines are often divided into the following groups:

• Nonsedating antihistamines. Because of the sleepiness experienced by almost half of those people taking early forms of antihistamine, many were unable to use this group of drugs. As a result, nonsedating (non–drowsy) forms of this medication were developed. Although early forms of nonsedating antihistamines were found to be unsafe, newer types (e.g., loratadine and fexofenadine) are considered both safe and effective at providing allergy relief without sedation. The early, unsafe types of antihistamines have all been removed from the market in the United States.
  
  
• Lightly sedating antihistamines. Some forms of newer antihistamines are found to cause sedation in only about 10 percent of the individuals who use the drug. For this reason, the U.S. Food and Drug Administration has decided that these forms of antihistamines – such as cetirizine – should by called “lightly sedating” instead of “nonsedating.”
  
  
• H1 antihistamines. These antihistamines are used to block the H1 receptors present in many types of tissue, thus preventing an allergic response. Because H1 receptors are the most common type of receptor, this is the most common and frequently used form of antihistamine. It is often used to effectively treat the symptoms of hay fever.
  
  
• H2 antihistamines. This type of antihistamine has fewer applications than H1 antihistamines because of the relative lack of H2 receptors in the body. Most H2 receptors are located in the stomach, making H2 antihistamines mainly effective at treating conditions such as stomach ulcers. This type of antihistamine has recently been used in combination therapies with H1 antihistamines to successfully treat conditions such as hives.