Hong Kong pharmacies are selling the drugs under the counter to mainland Chinese visitors who have lost faith in their own medical system and are dodging high prices, in another example of how demand from China can impact wider markets.
One shopper at a Hong Kong pharmacy in the bustling commercial area of Mong Kok told AFP he had travelled from the southern mainland city of Guangzhou to buy the breast cancer treatment Herceptin.
"My wife needs it to survive," the man, surnamed Li, told AFP.
"I will save more than 8,000 yuan ($1,280) per bottle if I buy Herceptin here. They may charge you 24,000 yuan for this on the mainland."
Customers from China buying the medication without the required prescription is "very common" said Chui Chun-ming, the chairman of the Society of Hospital Pharmacists of Hong Kong.
"This illegal trade brings them (pharmacies) a lot of money," he said, referring to the city's small independent high street businesses.
Chui says that 90 percent of the cancer drugs sold in pharmacies in Hong Kong go to mainland buyers as most local residents get them direct from hospitals or their own doctors.
With drug companies imposing supply quotas for different countries based on factors including population size, Hong Kong could fall short if demand continues from beyond its borders, Chui told AFP.
"The problem is Hong Kong is a very small city and the supply is very limited," he said.
"In a few years, it might turn out to be another 'infant formula' issue."
Chinese parents became distrustful of domestic milk brands after a huge 2008 scandal involving formula tainted with melamine that killed six children and sickened 300,000 others.
Their concern triggered a rush on milk powder, which saw shelves emptied around the world -- Hong Kong banned travellers taking out more than 1.8 kilogrammes of formula from March 1 this year.
Mainland demand has heaped pressure on Hong Kong's healthcare system in the past. Hospitals have banned mainland women who do not have local spouses from giving birth in the city to tackle increasing "birth tourism", fuelled by mums-to-be seeking better medical care and residency rights for their children.
Among their concerns are fears over the safety of medicines produced in China. Recent scandals include drug capsules made from toxic raw material derived from scrap leather and the busting of a ring peddling counterfeit tablets.
In 2008 a blood thinner called heparin, produced in China, was found to be contaminated and linked to dozens of deaths.
"People are not trusting their supply chains," Ben Cavender, an associate principal at the Shanghai-based China Market Research Group told AFP.
"They are worried that the drugs may be labelled incorrectly, or the quality is not that high, or the company decides to make it more cheaply in China than in the other countries, or they might be fake."
Buying or selling an anti-cancer drug without a prescription can lead to maximum fines of HK$100,000 ($13,000) and two years' imprisonment for buyers, salesmen and pharmacy owners, according to the Department of Health. A spokesman told AFP that it had boosted surveillance in response to mainland drug demand.
There were 24 cases in 2012 which led to convictions for the illegal sale of prescription medicines.
But despite the threat of prosecution, the trade continues.
When an AFP reporter visited four small independent pharmacies in Hong Kong and asked for 440mg of Herceptin, all offered to sell it without a prescription.
Tse Hung-hing, President of the Hong Kong Medical Association, said he feared the consequences of the uncontrolled sale of such strong medication.
"They (anti-cancer drugs) are more toxic. It is not like you are buying Panadol," he said.
Tse added that the organisation had reported the problem to Hong Kong's Department of Health, but had not seen a significant response.
Pharmacies tend to buy their cancer drugs from private doctors as a safer route than getting them direct from drugs companies, as direct orders are likely to be more closely monitored by the authorities, says Chui.
The Hong Kong branch of Roche, which manufactures Herceptin, said it had been "made aware" of reports of mainland visitors coming over to buy oncology drugs.
"We are committed to supporting the relevant authorities with any investigations," it said in a statement to AFP.
According to a 2012 report released by the China National Cancer Registry, the country sees 3.12 million new cancer cases every year.
From 2006 to 2010, the number of cancer cases in Hong Kong rose at an average rate of 2.7 per cent each year, four times more than the annual population growth rate, a report by Hong Kong Cancer Registry (HKCR) showed.
Chui believes the increasing pressure on Hong Kong's cancer drug supply could soon reach crisis point.
"The majority of community pharmacies in Hong Kong are owned by businessmen rather than professional pharmacists," he said.
"There is a high-probability that, in up to three years, the supply of anti-cancer drugs will become an issue."