How long myositis lasts varies depending on the cause:
Idiopathic inflammatory myopathies. Polymyositis and dermatomyositis are usually chronic (long-lasting) but typically improve after one to two months of treatment. Inclusion body myositis is also chronic. Since there is no reliably effective treatment for inclusion body myositis, symptoms usually worsen gradually over a period of years. If cancer is associated with the myositis, improvement may occur if the cancer is treated effectively.
Infectious myositis. Flu symptoms usually last three to seven days. In trichinosis, symptoms peak in about three weeks, and then gradually subside. For pyomyositis, your doctor will drain the abscess and you will be given antibiotics. Once the infection is gone, healing can start. Recovery can take weeks or even months in people with chronic illnesses.
Benign acute myositis. Patients usually recover within three to seven days.
Myositis ossificans. In some cases, the bony lump disappears on its own. However, this can take a few months. If the lump does not go away, it may remain indefinitely or your doctor can remove it surgically.
Drug-induced myositis. Symptoms usually disappear once you stop taking the drug. However, this can take weeks or months.