CVS Caremark, the drugstore and benefit management company, saw its shares fall more than 20 per cent on Thursday after it announced the loss of $3.7bn of prescription management contracts for 2010, and said that it has been under scrutiny by federal regulators.

The company said that it had been informed in August by the Federal Trade Commission that it was the subject of a non-public investigation into some of its business practices. It said it is co-operating with the FTC but gave no further details.