Prof Light argued: "Current incentives for research produce a few [drugs] that substantially improve patients' chances of getting better or avoiding death but a large number of barely innovative drugs each year.
"These new drugs of little benefit consume about four-fifths of all drug costs."
He accused companies of swamping drugs regulators with large numbers of "incomplete, partial, sub-standard clinical trials".
"The result is that drugs get approved without anyone being able to know how effective they really are or how much serious harm they will cause," he said.
When patients complained of adverse reactions, studies showed doctors were likely to dismiss them, he said.