WASHINGTON – Federal health regulators will require generic versions of the best-selling painkiller OxyContin to include recent formulation changes designed to make the pill harder to abuse.

The Food and Drug Administration said late Tuesday it would not approve any generic versions of OxyContin based on the original formulation because it “poses an increased potential for certain types of abuse.”

The FDA decision is a key victory for Purdue Pharma LP, which spent years developing a tamper-resistant version of the blockbuster pain pill. The company’s patent on original OxyContin expired Tuesday. Normally such expirations open the door to a slew of cheaper versions from generic drug manufacturers. However, the FDA decision effectively protects Purdue from lower-price competition by requiring generic companies to develop their own abuse-deterrent designs.

OxyContin has long been one the nation’s top-selling prescription painkillers with sales of more than $2.8 billion last year, according to prescription tracker IMS Health.

But drug abusers quickly discovered they could get a heroin-like high by crushing the pills and snorting or injecting the entire dose at once.

In 2007, Purdue Pharma and three of its executives paid $634 million and pled guilty to charges of misleading the public about the safety and addictiveness of OxyContin.

In April 2010, Purdue introduced a new version of the drug designed to be more difficult to crush, break or dissolve. On Tuesday, the FDA said it would update the drug’s labeling to reflect recent studies supporting those claims.