An Andover man pleaded guilty this week to supplying a drug that killed his girlfriend.

Seth Olson, 24, was charged with felony third-degree murder after his 39-year-old girlfriend, Krista Bogucki, overdosed on methadone Feb. 16. He had a prescription for the drug, but bought extra from a dealer, the charges said.

He gave Bogucki the methadone at 8:30 a.m., and a few hours later her breathing slowed.

He told police he checked on her several times during the day, the charges said. At nearly midnight, she stopped breathing and he called police.

The medical examiner said Bogucki had a high level of methadone in her system for somebody who wasn’t prescribed the drug, and it was a lethal dose, the charges said.

Olson pleaded guilty Wednesday and will be sentenced Oct. 17. The agreement calls for an 84-month sentence.