At about 3.30pm on Saturday, February 13, a man entered a chemists in Liverpool Road North in Burscough and walked behind the counter demanding drugs.
When a male staff member challenged the man, the offender pulled a six-inch stiletto knife from his pocket and forced the worker to open the drugs cabinet.
He then took 14 boxes of pain relief patches in a carrier bag before leaving the pharmacy.
The offender was described as being of stocky build, wearing a light coloured motorcycle helmet with a black visor, a black and white camouflage-style jacket and dark coloured combat-type trousers and grey gloves.
Police want to speak to the man pictured in connection with the incident.
Det Con Dominic Webster, of Skelmersdale CID, said: “We are appealing for information following a robbery at a pharmacy in Burscough.
“This was a particularly terrifying ordeal for the shop worker involved and we need to trace the offender immediately.
“The drugs taken included strong pain relief patches called Fentanyl, which can lead to serious medical complications if used incorrectly.
“We would urge anyone who recognises the description of this man, or has seen these drug patches in the Burscough area, to contact police.”
Anyone with information can contact police on 101 quoting crime reference number SB1600516.
Alternatively Crimestoppers can be contacted on 0800 555 111 or online at crimestoppers-uk.org.