Infection: Viral infection; Bacterial infection; Spirochete infection. E.g. Dengue fever; Trichinella infection
Influenza virus.
Drugs: Statins, Bisphosphonates, corticosteroids, ciprofloxacin, clofibrate, colchicine, chloroquine, emetine, aminocaproic acid, zidovudine, bretylium, penicillamine, drugs causing hypokalemia
Metabolic disorders: Vitamin D deficiency; Mitochondrial myopathy; Scurvy; Osteomalacia
Fibromyalgia
Endocrine: hypothyroidism; hyperthyroidism; Cushing’s syndrome; Adrenal insufficiency; Hyperparathyroidism
Polymyalgia rheumatica
Rhabdomyolysis (and anything that can cause rhabdomyolysis)
Myositis (muscle inflammation): e.g, polymyositis, dermatomyositis
Systemic lupus erythematosus (SLE)
Rheumatoid arthritis
Inclusion body myositis
Sarcoidosis
Scleroderma
Sjögren’s syndrome
Psychiatric (e.g. somatic manifestations of depression)
Domestic abuse; Crush injury
Lyme disease
Ehlers Danlos (Hypermobility syndrome)
HIV myopathy
Hypophosphatemia
Hypokalemia
Hypothermia
Prolonged immobility, eg, after a drug overdose.
Strenuous exercise (overuse) or heat stroke
Seizure
Severe volume contraction
Alcoholism
Muscular dystrophy, eg, Duchenne, Becker, limb-girdle, facioscapulohumeral (FSH); Myotonic dystrophy; Myotonia congenita
Compartment syndrome; Muscle infarction
Neuropathic
Chronic fatigue syndrome
Vasculitis
Sarcocystosis
Spinal stenosis
Diabetic lumbosacral plexopathy
Thinking through the Differential Diagnosis
Myalgias can be divided into diffuse (systemic) myalgias vs. Localized myalgias.
Muscle pain can come from Rhabdomyolysis, Myositis, or Myopathy.
You can also break down the differential diagnosis according to disease mechanisms, VINDICATED MEN.
Further Reading
https://www.uptodate.com/contents/approach-to-the-patient-with-myalgia