The average potency of pot has more than tripled in the past two decades, according to testing done for the federal government. This comes just over a year after Colorado and Washington legalized the drug and as many other states consider making it legal for medical or recreational use.

Scientists determine potency by measuring levels of THC, or delta-9-tetrahydrocannabinol, the main psychoactive ingredient that gives marijuana its “high.” And data from the University of Mississippi’s Potency Monitoring program found that the average potency of marijuana has jumped from 3.4 percent THC in 1993 to 12.3 percent THC in 2012. Scientists at the lab say they’ve seen samples as high as 36 percent.

This month’s “High Times” magazine, with a cover promoting “The Strongest Strains on Earth,” claims to have analyzed 15 strains of pot with potencies ranging between 25 to 28 percent THC. Marijuana near that strength can be bought at many legal retail shops and medical dispensaries across the U.S.

A quick bit of botany: The two main species of the Cannabis plant, Cannabis indica and Cannibas sativa, produce different kinds of highs. Most varieties of pot sold today are hybrids of both. Effects of either can include “altered perceptions and mood, impaired coordination, difficulty with thinking and problem solving, and disrupted learning and memory,” according to the National Institute on Drug Abuse. Newer strains of marijuana have different effects than those of the 1970s and 1980s, when THC averaged roughly 3 percent.

Indica is described as having a calming, relaxing, and narcotic effect, while sativa is said to have a more uplifting, stimulating, or “cerebral” effect.

Increasing the potency of the THC in marijuana plants means an increasing intensity of those effects.

As Colorado and Washington began selling legal marijuana, the NewsHour traveled to the Netherlands — the one nation that’s been openly selling pot for more than 40 years.

So what explains the rise in potency? Consumer demand is clearly one driver.

“I see people walk in all the time saying, ‘Give me the strongest thing you have,’” says Tim Cullen, co-owner of Evergreen Apothecary, which runs two retail marijuana stores in Colorado. “It’s bizarre … Can you imagine being in a liquor store and having someone say, ‘Just give me your strongest stuff?’ But for now, that’s what a lot of people seem to want.”

And marijuana growers have clearly been working to meet the demand. Robert MacCoun, a behavioral scientist at U.C. Berkeley who has studied drug policy here and abroad, calls it an “arms race,” in which growers strive to create the highest-octane varieties, and then bestow awards on themselves at the annual Cannabis Cup competition.

“The unfortunate aspect of this arms race is that they’re finally turning the drug into everything the U.S. government once said it was,” MacCoun says. “It used to be we could say the government exaggerated the threat of this ‘crazy weed,’ but these new potent strains belie that.”

Others say the U.S. government’s war on drugs has had the unintended effect of driving up potency.

“This is a problem of our drug policies, not a problem of the drug,” says Julie Holland, a psychiatrist, drug researcher, and editor of “The Pot Book: A Complete Guide to Cannabis.” Because marijuana is still considered illegal by the federal government, she says, growers and sellers have an incentive to pack more potency into a smaller volume. And that’s a problem, she adds:

“Because it’s illegal, you have no idea what you’re getting. If it were legal and could be taxed and regulated, it would be safer.”

While many consider pot far less harmful than legal drugs like tobacco and alcohol, it can pose risks to adolescents, pregnant women, and those with the potential for certain mental disorders like schizophrenia, according to the National Institute on Drug Abuse.

NIDA Director Dr. Nora Volkow believes higher potencies may exacerbate some of marijuana’s harms, triggering more feelings of paranoia and panic attacks in certain users.

“You become paranoid, you think that people are persecuting you and you get very, very anxious and you end up in the emergency room,” Dr. Volkow says. “[But] that usually resolves pretty rapidly.”

Of greater concern, says Volkow, is rising admissions to drug treatment programs where marijuana is cited as the main problem. This too she attributes to the drug’s potency.

“There really hasn’t been an increase in the number of people smoking marijuana,” Dr. Volkow says. “What has changed is the potency of the marijuana that individuals are exposed to.”

Critics of this argument, like the Drug Policy Alliance, argue that the growing number of treatment admissions have nothing to do with potency, and are instead a by-product of the war on drugs where those arrested for marijuana possession are diverted into treatment by the courts. The Alliance writes: “increasing admissions for treatment are a reflection of the criminal justice system’s predominant role, rather than increasing rates of clinical dependence.”

As to whether higher potency mean it’s more addictive, that’s unclear.

NIDA states that 9 percent of people who are exposed to marijuana will become addicted to it. While that’s a much lower rate than that of drugs like cocaine, which has a 17 percent addiction rate, or heroin at 23 percent, Dr. Volkow believes that more potency may be linked to higher addiction rates, though she admits there’s no good evidence connecting the two.

“[It’s] indirect evidence that there may be a linkage between the potency and its addictiveness,” she says.

A recent small study conducted in the Netherlands indicates that the way a user smokes marijuana – how frequently they take puffs, and how much of a cigarette they consume – might be a better predictor of marijuana dependence than potency, though these results haven’t been replicated elsewhere.

“What you worry about with dependence is, what does it look like if you abruptly stop?” Holland says. And withdrawing from marijuana, she says, is not nearly as difficult, or dangerous, as withdrawing from heroin or alcohol. “Abrupt cessation of alcohol is potentially lethal. Abrupt cessation of cannabis? … If you use pot every night to put yourself to sleep and then you don’t have any pot, then you may have trouble falling asleep. If you use pot every day to treat your nausea or your pain and you don’t have any pot, you’re going to be nauseous and in pain.”

But if marijuana is stronger, won’t users just consume less of it?

That’s what Columbia University neuroscientist Carl Hart says he’s documented in his work. Hart (also an author who has been critical of U.S. drug policy) has performed numerous cognitive experiments with marijuana. He believes that when experienced users are given stronger pot, they simply smoke less.

Hart says when marijuana is smoked, the drug’s effects are felt almost immediately, so users know when they’ve had enough. When test subjects in his lab are given marijuana cigarettes with relatively low THC levels, they’ll smoke the entire cigarette. “But when you increase the THC, half the cigarette comes back,” says Hart. “They don’t smoke it all.”

In fact, Hart believes there might even be a health benefit to stronger marijuana. “If you inhale less, it might reduce toxicity in the lungs,” he says.

But Volkow says not all users are able to ‘titrate’ – or modulate – their intake. “If you are a regular user and an expert on how you’re expected to feel with marijuana, you may be able to titrate,” she says. “But if you are not such an expert, how are you going to?”

Plus, modulating your intake of THC is also much harder if you’re eating rather than smoking it.

In Colorado, where retail shops have been selling legal pot for several months, edible marijuana products have become hugely popular. In these ‘edibles’ – which can feature marijuana-laced cookies or chocolates or gum drops — the entire dose of THC is contained in just a few bites, and when eaten, the drug’s effects take longer to register on the user. So even though it’s virtually impossible to overdose on marijuana, someone eating THC could end up consuming far more of the drug than intended.

In fact, a recent investigation by the Denver Post found “blatant misstatements” about the actual levels of THC in a range of edible products being sold presently in Colorado. Many of the products were far weaker than their labels claimed, but alarmingly, several of the products had THC levels almost 50 percent higher than advertised.