A new U.S. Food and Drug Administration proposal aims to provide clearer explanations for companies whose medicines do not win immediate approval, the agency said on Monday, but critics said the plan could cloud data for the public and stockholders.

Currently, if the FDA does not approve a drug, it can issue one of two letters - “approvable” or “not approvable” - depending on how close the product is to being acceptable.

Products that need substantial work, including additional clinical trials, are generally considered not approvable while those that need label updates and other minor changes are more likely to get the stamp of approvable.

But Monday’s proposal would replace both letters with a generically-named “complete response letter” that experts say is less likely to give investors and analysts an indication of how close the drug is to being approved in the future.

“The downside of this situation is that we ... are now captives to the description that a company will give to what is in the complete response letter,” said Ira Loss, an analyst for the Washington Analysis Corporation.

The content of both the current and the proposed letters are kept private but companies - and sometimes the FDA - often disclose which they have received.

Wall Street has generally been skittish about the FDA’s letters, and a “not approvable” letter can often knock a company’s shares lower.

Sidney Wolfe, head of Public Citizen’s Health Research Group, said the new letters may be “good for industry” but they could “obscure, from at least the public view, what is going on.”

The new letters, which are already used for biotech drugs, would still include specific steps a company must take in order to win approval.

“This new approach will provide a clearer and more consistent method for communicating to new and generic drug applicants about the status of their applications,” said Acting FDA Commissioner Lester Crawford.

Drug industry groups welcomed the proposal, which was expected as part of revisions to the Prescription Drug User Fee Act that allows the FDA to collect various fees from drug firms.

Critics say the law gives drug firms more control over the agency, leading to special treatment and speedier reviews that can overlook safety and increase recalls.

The proposed change would also include new timetables for companies to follow. Current rules require firms to resubmit their applications, which can extend the agency’s review up to 180 days.

Under the proposal, similar actions will either require set two- or six-month extensions based on the amount of additional work needed for approval.

FDA officials are seeking public comment on the new approval system until October 18. The proposed guidelines are available online at http://www.fda.gov/OHRMS/DOCKETS/98fr/04n-0267-npr0001.pdf