Although drug-induced myopathies are not uncommon, they are difficult to identify in clinical practice.
Several drugs can be myotoxic both alone and in combination with other drugs.
Weight loss and elevation of the serum lactate level may herald the onset of zidovudine myopathy. Zidovudine can cause myopathic symptoms after one year of administration. In a follow-up study of 20 patients with zidovudine myopathy, four to six weeks after zidovudine was discontinued, myalgia resolved, strength improved or normalized, and spontaneous activity on the EMG decreased or disappeared. The unique histologic features of zidovudine myopathy are ragged red fibers, suggestive of mitochondrial abnormalities, and many cytochrome c oxidase-negative fibers. Although the newly introduced protease inhibitors have not been associated with myotoxicity, careful observation is warranted to determine whether it occurs after long-term use.