A pharmacist has admitted to drugging his colleague 23 times with prescription drugs because she rebuffed his sexual advances.

On Tuesday, Yan Chi "Anthony" Cheung pleaded guilty in Waverley Local Court to one count of poisoning to injure or cause distress or pain.

Cheung, 35, and his victim, Pamela Leung, 26, have been friends since they met at the Surry Hills Chinese Presbyterian Church in 2010.

In 2015, Cheung encouraged her to take a position at Pharmacy@UNSW, based on the university campus, and started making sexual advances towards her when she began working with him.