VANCOUVER -- For Hugh Lampkin, fentanyl's surge to all but replace heroin on the Vancouver drug scene calls to mind a curious image: a rainbow.

"Traditionally, heroin comes in about four different colours," said the longtime drug advocate, describing a bland palette of beiges, browns and blacks.

"Well now you're seeing multiple colours, like colours of the rainbow: green and pink and orange and white. ... Right away, when you see these colours that's a pretty good indicator that it's fentanyl that you're doing."

As government data tracks a spike of fentanyl across Canada, people who use illicit drugs in Vancouver's Downtown Eastside say there is virtually no heroin left on the street after it has been pushed out by the cheaper and more potent fentanyl.

Martin Steward of the Western Aboriginal Harm Reduction Society said fentanyl's takeover is evident by how easily people are overdosing on small amounts of what is being sold as heroin, and simply by people's physical response to the drug.

"I know people who use heroin and they'll inject what they normally do. And the next time they'll do exactly the same thing of what they think is heroin and they're out. Like, they're going under from it," Steward said in an interview, referring to an overdose.

"They're using the same thing, the same product, but getting a different result. That's a forerunner for me to see that it's not heroin."

There have been 256 fatal overdoses from illicit drugs in the first four months of this year, already more than half the 480 that occurred for all of 2015. Fentanyl's connection to those deaths has been surging at a staggering rate.

The B.C. Coroners Service reported last week that the presence of fentanyl in cases of illicit drug overdose deaths rose from a third in 2015 to nearly 50 per cent so far this year.

Speaking anecdotally, Lampkin said he doesn't believe anyone in Vancouver has used real heroin in more than a year and that many users don't appear to be aware of it.

He's observed overdose victims needing three full vials of the overdose-reversing drug naloxone to recover, he said.

"I think it's not so much as they're moving to it as a case of not having any choice," said Lampkin, who sits on the board for the Vancouver Area Network of Drug Users.

"The people who are controlling the supply, they're passing off what should be heroin as fentanyl because of the close proximity of the high."

Vancouver police report heroin-related drug seizures and criminal charges in the city have remained relatively stable over the past five years, but Lampkin said drugs are only tested when charges are laid or usually in the event of a fatal overdose.

Sgt. Darin Sheppard, who heads up a British Columbia RCMP division that investigates organized drug crime, said that while heroin is still present in the province, fentanyl is increasingly taking over the market.

"It's a growing trend," he said, pegging 2014 as the first year fentanyl was noticed in a significant way.

Mark Haden, a public health professor at the University of British Columbia, draws a parallel to alcohol prohibition, which he said led to stronger, more concentrated booze that was often toxic.

"Dealers will always want small packages. That's the natural process of drug prohibition," he said, dismissing the war-on-drugs policy approach taken by governments as shortsighted and ineffective.

There are multiple explanations offered for the rise of the dangerous opioid, centring on its low production cost and the simplicity of smuggling it across the border in its a compact, concentrated form.

Jane Buxton with the Centre for Disease Control said money plays a key role in fentanyl's upward trend line.

"Whoever is importing or selling drugs, they're doing it presumably for a profit and therefore if there's a substance that is easy to access and cheap, and can be sold for a great profit, that's what's going to be focused on," she said.

The manufacturer of the prescription opioid OxyContin designed a tamper-resistant version of the prescription drug that becomes inert when meddled with, making it impossible to grind and snort, for example.

The effectiveness in disabling OxyContin as a drug source has in turn contributed to a spike in black market opioids, Buxton said.

Still, it's difficult to know exactly what is happening on the ground without effective and timely data collection, she added.

Michael Parkinson of the Waterloo Region Crime Prevention Council in Kitchener, Ont., lamented that no province, territory or the federal government gathers real-time data on opioid overdose fatalities.

That is seriously hampering their ability to craft fast and effective responses to drug crises, he added.

"(With) other causes of accidental death, for example influenza, we know how many people died or were hospitalized last week," said Parkinson.

Alberta and B.C. now have more up-to-date numbers on fentanyl overdose deaths, he said, but other opioids aren't included.

"It's an international mystery. It really is. It's scandalous," Parkinson said, pointing out that there have been 4,984 deaths in Ontario due to opioids over a 13-year period.

"We get three people dropping off from anaphylaxis and it's all hands on deck," he added. "That hasn't happened with opioid overdoses."