A Massachusetts State Police trooper on medical leave was arrested yesterday at a Worcester hospital and charged with robbing a Natick pharmacy of OxyContin on the day after Christmas, police said.

Christopher Shields, 41, of 33 Dinsmore Ave., Framingham, a 16-year state police veteran, was ordered held on $25,000 bail yesterday in Natick District Court by Judge Sarah Singer.

Shields is charged with armed robbery and possession of a Class B substance, court records show. OxyContin is a Class B substance under state law.

According to records, Shields was arrested on Thursday at Sudbury Farms grocery store in Sudbury for shoplifting DVDs, magazines, soda and chips. Sudbury Police would not answer any questions about the arrest.

Sudbury Police didn't realize he was connected to the Natick armed robbery, and Shields was released on personal recognizance from Framingham District Court on Friday, the Middlesex district attorney's office said.

After looking at his picture later, Sudbury Police matched Shields to the description of a man who robbed the Natick CVS on West Central Street at the corner of Speen Street on Dec. 26, Middlesex District Attorney Martha Coakley said at a Cambridge press conference.

"The detectives in the MetroWest area, especially in cases such as these, are in contact with each other," Coakley said.

The Essex district attorney's office will handle the investigation because for a few years during the early 1990s, Shields worked as a homicide investigator in the Middlesex district attorney's office.

Shields was arrested early yesterday at the AdCare Hospital in Worcester, Natick Police said. The hospital is a drug and alcohol rehabilitation center that Shields entered voluntarily on Monday.

State police Col. Thomas Foley said at the press conference that Shields was suspended without pay.

"It's always difficult when anyone in law enforcement, especially a police officer, is arrested in a manner like this," said Foley.

"It's difficult for any police officer, even the state police, because we're proud of our agency. The state police are made up of human beings with human frailties, but he made his bed, and he'll have to lay in it," Foley said.

There were several robberies of OxyContin in MetroWest last year, including two in Natick, and others in Framingham, Wayland, Marlborough and Milford, Shrewsbury and Ashland.

Last month, a man robbed a CVS on Franklin Street in Framingham at the corner of Mount Wayte Avenue.

Neither Coakley nor Natick Police Lt. Nick Mabardy would identify Shields as a suspect in any other robberies of OxyContin, a drug that can create a heroin-like high when crushed and snorted or injected.

"At this time, all I want to say is it's a continuing investigation," said Coakley. "Throughout that region and throughout the state, there have been several similar robberies."

Prosecutor Jennifer Lennon said in court yesterday that Shields was identified by police as the suspect who entered the Natick CVS and approached the pharmacy counter.

"She (the pharmacist) was on the phone, and he put a bottle of Nyquil on the counter as if he had a question," said Lennon. "She stated she would be a while, and he said he'd wait. When she got off the phone, the suspect said he wanted all of the OxyContin, and pointed to a note that said, `I want all of your OxyContin.'"

The suspect pulled his jacket to the side and showed the pharmacist a gun in his waistband, Lennon said. She gave him three open bottles containing an unknown amount of the powerful painkiller.

Lennon recommended $50,000 bail, but defense attorney Michael Brennan said that was excessive.

"He's a lifelong resident of Framingham," Brennan said. "Up to a year ago, he lived in Framingham with his family. For the last year, after he was separated, he has lived in an apartment in Framingham.

"I've known him professionally and personally for the past 12 or 13 years. I would think for anyone who has known Mr. -- Trooper -- Shields, this will come as a big surprise," Brennan said.

Shields had an exemplary state police work record and was never disciplined, Brennan said. He recommended $1,000 bail, with Shields entering a treatment facility.

Lennon argued the crime was too serious for so little bail.

"My concern is, this is a dangerous crime -- he's going around showing a gun," she said.

Singer ordered $25,000 bail, and set as a condition, if Shields made bail, that he would go to a secure drug treatment facility, or a less secure one if he wears a security bracelet.

Shields' most recent assignment was patrolling the Massachusetts Turnpike out of the Weston state police barracks, Foley said. Shields has been on medical leave since October 2001 after injuring his knee and back in a fall at work.

"He was a good investigator," said Foley. "I think if you ask any trooper who worked with him, he's a pleasant personality and easy to get along with, but in the last couple of years, he has had some personal problems.

"We're not any different than anyone else," Foley said. "It's just the nature of the business that we get more attention."