Drugs whose names start with the letter D include Daraprim, an antiparasitic drug used to treat toxoplasmosis; Decadron, a steroid used to treat inflammation; and Demerol, a pain reliever. Daraprim is available only by prescription, while Decadron and Demerol are available over-the-counter. Decadron is used to treat a variety of conditions, including allergies, asthma, and arthritis. Demerol is used to relieve moderate to severe pain. It is important to consult with a healthcare provider before taking any medication, as some drugs may interact with other medications or have side effects.