One-fifth of American adults — 50 million people — suffer from chronic pain, defined as pain experienced most days or every day during the past six months. Conditions include migraines, sciatica and gastrointestinal disorders, as well as shoulder, knee and elbow pain. Back and neck pain, too, affect up to 85 percent of adults at some point in their lives and are among the most common reasons for doctor and hospital visits. Chronic pain results in more than $500 billion each year in direct health-care and disability costs and lost productivity. Roughly half a million Americans have died over the past two decades after overdosing on opioids, commonly taken in a desperate quest for pain relief.