A new analysis finds that the U.S. system for reporting side effects caused by prescription drugs may be producing an unfortunate side effect – incomplete information about injuries. And the reason appears to be haphazard reports filed by drug makers, according to the Institute for Safe Medicine Practices, a non-profit that tracks drug safety issues.

The FDA’s Adverse Event Reporting System is the primary vehicle for gathering information about side effects that may be caused by prescription medicines. Consumers and physicians...