Police charged a 31-year-old Port Kembla man after he presented the forged script to a Warrawong pharmacy, about 1.30pm on Tuesday.

Police were notified after a suspicious pharmacy staffer phoned the prescribing doctor, who denied writing the script.

The man allegedly stole a blank prescription pad leaflet from a Wollongong methadone clinic, mid-consultation.

Police say he then wrote a prescription for Kalma, commonly known as Xanax.

The man has been charged and will appear before Port Kembla Local Court.