GP data can give real-world views of a drug's value, said Professor Paul Wallace, a north London GP and director of the Primary Care Research Network at the NHS National Institute for Health Research.
Last week researchers writing in the BMJ called for more studies into the cost-effectiveness of preventive therapies outside clinical research settings.
Professor Wallace said: 'There's a tremendous potential to develop work looking at effectiveness in real life - certainly it can be done.'
He said practice systems could allocate patients to drugs to test effectiveness.
Where two or more drugs would be equally appropriate, the system could randomly allocate a patient to receive one option, allowing researchers to track outcomes in reality.
Professor Wallace said UK bodies such as NICE should do more to include pragmatic effectiveness data in its recommendations.
But rejecting all clinical trials as poor evidence for effectiveness risked 'throwing the baby out with the bathwater', he said. 'Pragmatic trials do build in requirements to show effectiveness,' he said.
Finnish researchers writing in the BMJ said governments and drug approval bodies should promote and fund cost effectiveness trials.
They called for true comparative effectiveness research to become routine. 'Unless this is done, the important question whether preventive pharmacotherapy is cost effective will remain unanswered.'