Cannabis can regrow brain cells, so the argument goes. It's become a favorite claim for supporters of medical weed, but there's a catch.

It may be a myth.

According to researchers at the University of California at San Francisco, brain cell growth — or "neurogenesis" — does not occur in the adult human hippocampus. This study seemingly flies in the face of the past two decades of research claiming the hippocampus could regenerate lost cells. In other mammals, such as rats and mice, the hippocampus is believed to direct neurogenesis in other parts of the brain as well, but humans probably cannot tap into this Wolverine-like ability.

"It's great to say, 'In the rodent brain, exercise increases neurogenesis' or 'CBD increases neurogenesis,'" Michele Ross, PhD, told MERRY JANEby phone. "But if it doesn't happen in the [human] adult brain, it's not worth bringing it up. It's completely irrelevant."

Ross is the executive director of IMPACT Network, a non-profit medical cannabis research organization. Although she was not a part of the UC-San Francisco research group, she studied neurogenesis and drug addiction in rodent models during her graduate and post-doc studies at the University of Texas Southwestern Medical Center and the California Institute of Technology, respectively. Since then, she has strongly campaigned for cannabis legalization, but she's no longer convinced weed can grow new brain cells.

"It's not a valid argument after this paper," she says. The paper "could be disproven in the future, but there are a lot of great people working on this study. It will probably take a long time before anyone can get the brain samples to follow-up."

The UC-San Francisco researchers looked at post-mortem brains donated to science from 12 deceased adults. They analyzed certain molecules that only form in new neurons, but they didn't find any in the donated brains. "Even in our best-preserved samples," one of the researchers, Mercedes Paredes, told The Atlantic, "we didn't see any evidence of neurogenesis."

But not so fast says one renowned scientist.

"Keep in mind, this study only dealt with the hippocampus," says Robert Melamede, PhD. "There are other studies showing neurogenesis in other regions [of the brain] not addressed by this study."

Melamede is a retired University of Colorado professor who was one of the pioneers of the molecular biology field. He co-founded the biotech firm Cannabis Science Inc., and he taught the world's first university-level accredited medical marijuana course until 2012. Although his specialty is DNA damage and repair, his expertise intersects with all facets of the human body, including the brain.

To counter the no-neurogenesis claims, Melamede pointed to Alzheimer's patients who regain some functionality and memory after CBD treatments. One example is his own brother, whose condition grew worse until their sister encouraged him to try cannabinoid therapy.

"There are times that he's normal," Melamede says. "We know that with advanced dementia, you have significant neuron loss. Here, we have a man who was a vegetable now returning to health."

Melamede continues: "Either this man has gotten better without neurogenesis or he has neurogenesis. I believe he has neurogenesis. And this happened under suboptimal conditions, but regardless, he's made incredible strides where everybody notices it."

By "suboptimal conditions," Melamede is mainly referring to diet: he questions the recent neurogenesis study because it did not consider the brain donors' eating habits. A diet rich in omega-3 fatty acids (think fresh fish, olive oil, and avocados) contributes to brain health, and the American diet (think processed food) typically lacks omega-3s. These fatty acids are critical to understanding neurogenesis in humans because they're the precursors, or building blocks, for our endocannabinoids – otherwise known as our body's "natural marijuana."

Melamede notes that "16 percent of the brain's weight is DHA, a fatty acid your body doesn't make. If you're not eating the right shit, then your brain cells are dying, and you're not going to have neurogenesis."

The debate over whether full-grown humans can make new brain cells remains one of the most controversial topics in modern neuroscience. The verdict will likely remain out until new studies – and new measuring techniques – become available.

"We can't cherry-pick the medical benefits that we want to use for our [pro-cannabis] arguments," says Ross. "If [neurogenesis] is not a real benefit, we have to drop it, just like we have to drop the baloney [anti-cannabis] propaganda studies."

Whether weed stimulates neurogenesis or not, cannabis still provides several health benefits acknowledged by both Ross and Melamede. Ross says the new study, if it does debunk adult neurogenesis, shouldn't detract from cannabis's other healing properties.

"Cannabinoids are great for the brain," she says. "CBD's anti-inflammation reduces stress and improves learning and memory."