Several different forms of allergy medicine are available. Some drugs, like antihistamines, are taken by mouth. Others, such as nasal steroids and cromolyn sodium, are taken as a nasal spray. Decongestants are available as pills, tablets, caplets, and as nasal sprays or drops. While some allergy medicines require a prescription, others can be purchased over the counter. As with any drug, these medicines have potential side effects and precautions to be aware of, so be sure to take them exactly as directed.