Pain sufferers are being slugged almost $20 in south-west pharmacies for a 24 pack of the pills – a 100 per cent increase at some outlets.

A number of other south-west residents have reported being told by their pharmacist they no longer stock the painkiller.

The price hike has resulted in a number of complaints to Health Minister Greg Hunt, who has referred the “alleged pricing practice” to the Australian Competition and Consumer Commission for investigation.

“There is no reason for the costs of these products to increase,” a spokesman for the minister said.

“It is important to note that generally speaking businesses are free to set prices as they see fit, as long as they do so independently of their competitors and any claims are substantiated and do not mislead consumers."

The maker of Nurofen Plus, Reckitt Benckiser, has admitted to a price increase for its product.

"The regulation change placed Nurofen Plus in a very different commercial environment, and we have had to adjust our pricing model accordingly,” a Nurofen spokeswoman said.

Pharmaceutical Society of Australia president Shane Jackson said that as a result, the price of a pack of Nurofen Plus had increased from about $11 or $13 to closer to $18.

“The pharmacy hasn’t been the culprit in the price increase, largely the prices have been set by the manufacturers,” Dr Jackson said.

Since February 1, sales of low-dose codeine have plunged almost 90 per cent (compared to the year before), according to data from the Advantage Pharmacy Chain.