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 expires 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. A time-release version of the narcotic oxycodone, the pill was hailed as a breakthrough treatment for severe chronic pain when it was approved in late 1995.