US government advisers have urged regulators to reject an experimental painkiller from King Pharmaceuticals and Acura Pharmaceuticals.

The Acurox pill was designed to discourage abuse by adding the vitamin niacin to the painkiller oxycodone. The companies said people who take multiple pills would get high doses of niacin that could cause uncomfortable flushing.

Members of a Food and Drug Administration (FDA) advisory panel praised the drugmakers for trying to reduce improper use but said they had not shown niacin would be a strong deterrent.

The FDA usually follows panel recommendations when deciding whether to approve new medicines.

King shares closed down 2.8 per cent at $11.10 on the New York Stock Exchange. Acura shares were halted on the Nasdaq while the panel met.-Reuters