CONCORD, N.H. — A pharmacist has pleaded guilty in New Hampshire to tampering with customers’ pain medications, using syringes to withdraw the drugs and replacing them with saline.

The US Attorney’s office in New Hampshire says 65-year-old Thomas Kellermann, of Bedford, pleaded guilty Wednesday to tampering with a consumer product and obtaining a controlled substance by fraud. He’s scheduled to be sentenced in May.

Court documents say Kellermann worked at a pharmacy and began abusing narcotic pain relievers in 2011. He went on medical leave in 2012 but reportedly returned to the pharmacy after business hours and on weekends to take the drugs and replace them with saline.

Prosecutors said a pharmacy employee detected irregularities in certain vials of drugs, which were analyzed by an independent laboratory and found to be substantially below their labeled strength.