As Californians prepare to vote on legalizing recreational marijuana, innovations in cultivation practices are pushing the plant’s psychoactive properties to unprecedented heights — packing a powerful punch that delights some but alarms others.

Levels of the chemical that produces a high — known as THC — used to average 3 to 4 percent. But now — because of improved breeding, growing, harvesting, packaging and shipping techniques — it’s rare to find THC content below 20 percent.

And some varieties — such as the award-winning “C. Banana,” grown by medical marijuana grower Utopia Farms of Santa Cruz — can reach a stunning 35 percent. For the unprepared, it’s like ordering a rosé spritzer and getting Jack Daniel’s.

But potency is also a serious concern as California heads toward a new world of legalized pot, potentially worsening marijuana-related illness and dependency. New consumers and longtime potheads may find themselves dangerously high before they know it, and unequipped to deal with the results.

var _ndnq = _ndnq || []; _ndnq.push([’embed’]);

“This marijuana is not like the old marijuana,” said Dr. David Smith, who in 1967 founded the Haight Ashbury Free Medical Clinic. He now treats cannabis-dependent teenagers at Marin County’s Muir Wood Adolescent & Family Services.

Related Articles Here’s how THC makes you “high”

Is California ready to legalize pot?

Why Prop 64 is about more than just smoking marijuana

Prop. 64: California farmers divided on legal bud

Watch: How California can learn from Colorado’s marijuana laws Proposition 64, a detailed 62-page ballot measure, describes the laws, regulations and taxes that will govern the recreational use of marijuana. If passed, consumers would grow their own, buy from a retail shop or visit a private smoking lounge, rather than getting a “medical marijuana” card or buying it on the black market.

But the proposition doesn’t address the thorny issue of potency.

Some say legalization will create better-educated consumers and a broader array of lower-octane products, ending the current “bigger-is-better” THC arms race. Others say it could push potency even higher, as horticulture improves. They’re particularly worried about consumption of extracts, called “dabbing,” with THC levels that can reach 80 percent.

Colorado, Oregon and the Netherlands are debating measures to limit potency, efforts that draw the ire of industry groups. Some policy experts say taxation is a better tool for regulating the chemical tetrahydrocannabinol (THC).

“If I were designing a legalization ballot initiative, I’d be inclined to propose taxing marijuana by its THC content,” said Robert MacCoun, a behavioral scientist who teaches at Stanford Law School and studies drug policies.

Instead, Proposition 64 would impose a 15 percent excise tax on all marijuana sales.

At Utopia Farms, whose “C. Banana” won top honors for “Highest THC” in three competitions last year, achieving potency is a science, said co-founder Kaiya Bercow.

The product of generations of selective crosses of high-THC plants, its flowers glitter with trichomes, the tiny resin glands that produce the molecule. It is grown indoors to prevent wind stress and contamination. Fully sealed rooms are climate-controlled to match the growth cycle of the plant, with fluctuating temperature, humidity and carbon dioxide levels. Its soil mix is customized; so is its fertilizer.

Bright green, the harvested flowers are promptly packaged in sealed glass jars and kept cool to prevent THC decay during shipping and sales. Light, heat and air degrade THC.

“It is connoisseur quality, an artisanal flower,” said Bercow, who graduated summa cum laude from Boston’s Tufts University before co-founding Utopia Farms.

Unlike alcohol or pharmaceuticals, the effects of a cannabis overdose — while frightening and miserable — is not lethal.

There are no studies that link high potency pot to illness and dependency, but some physicians are seeing more cannabis-using patients seeking treatment for anxiety, panic attacks and paranoia. After marijuana was legalized, in Colorado, calls to the Rocky Mountain Poison and Drug Center increased more than five-fold, from 45 to 238 between 2006 and 2014. The rate of marijuana-related emergency department visits jumped 67 percent, from 153 to 256 per 100,000 visits.

Doctors say those greatest at risk are people with mental health problems and aging baby boomers — “Geezer rockers, the ‘oldie but goodie’ group who are now responsible citizens,” Smith calls them — with nostalgic memories of big baggies filled with Panama Red or Acapulco Gold.

That weak, field-grown weed, whose THC content plummeted while hung from hot roofs and shipped in the back of smugglers’ dusty trucks, bears no resemblance to today’s high-tech weed, he said.

“We had a Rolling Stones concert where a software millionaire paid thousands of dollars for front row seats,” Smith said. “His daughter passes him a joint and he takes a couple of hits — then thinks he’s having a heart attack. His eyes are red. His heart is pounding.”

At San Jose’s Good Samaritan Hospital, “we are admitting two to three people every weekend with psychosis from marijuana,” said Dr. Jerry Callaway, an internist and addiction medical expert. “It is more prevalent because now it is more available and has become far more powerful.”

Several factors are boosting marijuana’s potency. For starters, it’s finally possible to test for THC. Analytic labs such as Berkeley’s Steep Hill Labs and Santa Cruz’s SC Labs issue certificates based on chromatography and spectroscopy findings.

Another is the growing influence of “pot critics” and national contests such as the Cannabis Cup and Emerald Cup. High-THC products earn the high scores and publicity that are critical to sales.

“It’s just human nature to want to push limits, like extreme sports, going higher, faster and farther,” said longtime cultivator and consultant Kyle Kushman of Woodland Hills, whose work has earned 13 national awards.

He also blames illegality, comparing it to Prohibition, when it was much more profitable to make spirits than beer. “If you might go to jail, you’re going to make the most of the little closet space you have,” he said.

With legalization and more experience, THC content could climb another 3 to 5 percent, reaching 35 to 38 percent, predicted Bercow.

He said a legal market also could introduce “niche brands” that are lower in mind-altering THC but higher in other desirable qualities, like taste and fragrance.

“It would be like fancy coffee shops or craft breweries, where consumers want unique products,” he said.

Given the modern-day powerful strains of pot, addiction expert Smith is braced for two outcomes if Californians legalizes marijuana: one bad, the other good. There will be more people needing treatment, he predicted. But he welcomes a reduction in arrests and convictions.

If legalized in a diverse state of 38 million people, “it will be a natural large-scale pharmacological experiment,” he said. “That’s never been done before.”