Novartis AG recently discovered that a drug it sells for a group of very rare diseases could be used to treat a much more common ailment. There is just one problem: its $16,000-per-dose price tag.

The drug, called ACZ885, is already sold under the brand name Ilaris for certain rare inflammatory disorders affecting a very small number of people. But a recent clinical trial suggests it could also reduce the risk of serious complications like strokes in people who have suffered a heart attack.

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