1. A pharmaceutical composition comprising an effective amount of methylnaltrexone or a pharmaceutically acceptable salt thereof, about 0.4 mg/mL of calcium ethylenediaminetetraacetic acid (EDTA) disodium, and about 0.1 mg/mL to about 0.8 mg/mL of glycine in an aqueous solution, wherein the solution has a pH of about 3 to 4.