The majority of samples tested came from recreational-pot merchants. Under Colorado law, recreational weed must be tested for potency. Some medical-pot sellers voluntarily provided samples to LaFrate. Colorado does not require pre-sale testing of medical marijuana. LaFrate did not analyze any edibles.
"Really, there is very little difference between recreational and medical in terms of the THC-to-CBD ratio, at least at the aggregate level," LaFrate said.
What does that mean for buyers? There may be little difference in how various strains make users feel, even though some people claim one type induces relaxation and another hikes alertness, LaFrate said.
Three decades of cross-breeding pot strains — done to meet a demand for stronger weed — generally elevated THC and decreased CBD in many marijuana varieties, LaFrate said.
"These samples are representational, I think, of what's happening here in the state and, probably, across the country," LaFrate said. "Because most of the new states coming online with medical or retail marijuana have people from Colorado coming in to set up those markets.
"We found there's a tremendous amount of homogeneity within the genetics, at least as far as potency."
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But some legal weed producers have launched new breeding projects, using different genetic combinations to boost CBD content, said Sean Azzariti, a cannabis advocate in Denver.
Azzariti also champions contamination testing as "an integral part of our industry."
"I personally am very excited to see technology in testing continue to advance. You would be very hard pressed to find a garden that hasn't at one point had some sort of issue, whether it's an infestation, microbial problems," said Azzariti, an Iraq War veteran. He uses cannabis to help treat post-traumatic stress disorder.
On Jan. 1, 2014, he became Colorado's first buyer of legal weed.
Meanwhile, pot-legalization opponents are using LaFrate's findings to compare retail weed to food raised or grown with genetically modified organisms or GMOs. And pot foes continue to link the rise of the marijuana industry to the long-ago advance of Big Tobacco.
"This study is further evidence that Colorado legalization is not working. It proves that even under government control, there's no way to ensure marijuana is free of bacteria and chemicals," said Kevin Sabet, president of Smart Approaches to Marijuana (SAM).
"This shows that marijuana is a GMO product just like other products sold by big business. And just like other industries, now you have a big marijuana industry determined to hide these findings from the public. Where is their outcry? Where are the promises to change the way they do business?" Sabet said. "I won't hold my breath. For years, the tobacco industry did the same thing. Welcome, America, to Big Tobacco 2.0 — Big Pot."