An aqueous nitroimidazole composition comprises metronidazole, tinidazole or a combination thereof at a concentration greater than the solubility of the free base form of the nitroimidazole in water at 20° C., and a nitroimidazole crystallization-inhibiting amount of at least one organic acid. The organic acid preferably is a lower alkyl carboxylic acid (e.g., acetic acid), a polybasic acid (e.g., citric acid, tartaric acid, malic acid, polyacrylic acid, and the like), or a combination thereof. The composition can further include a thickening agent, to form a gel. The composition is free from organic co-solvents, water-soluble vitamins, and cyclodextrins; and free from nitroimidazole crystals at an ambient temperature of about 20° C. Methods of preparing the composition are also described.