We’ve found that co-payments do not resolve behavioral hazard. They make it worse. They reduce the use of a drug that is already underused.

A recent experiment by a team led by Niteesh Choudhry, a professor of medicine at Harvard, quantifies the problem. The experiment involved nearly 6,000 patients who had just suffered a heart attack, and were prescribed drugs known to reduce the chance of another one — statins, beta-blockers, angiotensin-converting-enzyme inhibitors or angiotensin-receptor blockers. Half had their co-pays for these drugs waived; the other half paid the usual fee.

As expected, more people in the zero co-pay group took the drugs, and their health improved. Those in the zero co-pay group were 31 percent less likely to have a stroke, 11 percent less likely to have another major “vascular episode” and 16 percent less likely to have a myocardial infarction or unstable angina. None of these benefits came at a net monetary cost. The insurers did not spend more in total. By some measures, they spent less.

Behavioral hazard affects all of the drugs listed above. They are all highly effective, and yet adherence to taking them is a problem. This is a major financial issue.