.... and who fucked the public over about it!
... the main question: does it cause any significant brain damage?

Contrary to the belief of most citizens, the marijuana industry leaders/researchers are not the ones who initially tested marijuana for damage to the human body.

It all started with a man named Harry Anslinger (born 1892). In 1930, he worked for the Treasury Department, which had just created the Federal Bureau of Narcotics. The FBN mostly had a purview over opium related products (heroin, morphine), and at this time they did not focus on marijuana at all.

Fast forward to 1934.
The heart of the Great Depression. Federal cuts, slashing spending. There are rumors of the Narcotics bureau being eliminated completely. Cue Anslinger needing a reason to keep bureau relevant. Suddenly, he was lobbying Congress about a new “menace to society, that requires immediate action in the form of a well funded Bureau of Narcotics”, that menace was marijuana. (FYI, prior to 1934, he said he wouldn’t go after Marijuana because there was ‘no point’, as it was widely used at this time). This was the beginning of many politicians going after Marijuana as a scapegoat, namely using the verbiage “where we find crime, we find marijuana and alcohol. The difference being marijuana has a power of motivation, that causes crime”.

Think about reading headlines in newspapers “Marijuana causes blood lust”, “Marijuana causes homicidal massacres”. Lawyers using the “marijuana insanity” defense.

But that’s all referring to crime in relation to marijuana. Which is where we first received the stigma that remnants today, that marijuana is “bad”. This leads us to the studies that were first completed, into whether marijuana is dangerous or not.

The U.S. Army was one of the first to complete studies into marijuana, in 1925. They were concerned that use by soldiers could lead to a breakdown in military discipline. They created a committee of lawyers, public health officials, mental health experts, etc. The committee found “no evidence of negative health effects from marijuana”, and that people thinking it caused insanity had “little basis in fact”.

The next relevant study was from the New York Academy of Medicine. Anslinger specifically requested this study be done by NY AMA. This was in the late 30’s early 40’s. They held clinical studies and administered the drug in various settings. Their findings were that it produced a “euphoric state, with feelings of well being, contentment, sociability, mental and physical relaxation, and ended in a feeling of drowsiness.”

This led the Federal Bureau of Narcotics to state that “anyone who conducts marijuana research without the FBN’s approval, will be arrested and tried on federal charges.”

Let’s fast forward to Nixon and the “Controlled Substances Act”.

Under this act, they placed drugs in 5 categories to determine their likelihood of danger/abuse to the American public. The only problem? They didn’t know where to put Marijuana.

The Controlled Substances Act created the “Presidential Commission on Marijuana and Drug Abuse” specifically to determine the placement of pot. Thus began, the Shafer Commission.

Nixon (before hand picking the panel members of the Shafer Comm.) made it clear what they wanted the Commission to find. This is where the “gateway” theory was born. Attorney Gen. John Mitchell stated “a kid gets heavy into pot. Gets less of a charge from it, and moves into the harder stuff. We have to get proof that it creates this dependency”. Not “we have proof”, “we have to get proof”.

The Commission began in 1971. One year later, they gave their full report to Nixon. The hand-picked-by-Nixon Commission, debunked every negative thing that the Government had said about Marijuana for the last 40 years. No fatalities were found. No brain damage was found. No gateway theory was supported. The report stated that if any drug was a gateway drug, it was tobacco, followed closely by alcohol.

It found that not only should the drug not be a schedule I, but that it shouldn’t be scheduled at all on the Controlled Substances Act. They proposed that it should be decriminalized.

Nixon disowned the Commission. He created the DEA after reading the report (1973). Marijuana was subsequently scheduled as I, along with meth, and heroin.

In short, no, marijuana doesn’t cause brain damage. It’s been the scapegoat of many a politician, and has been proven to be better for you than alcohol.

NOTE: I can assure you that I am unbiased in this answer, namely because I don’t smoke or consume marijuana.

Sources:
The Shafer report
Bruce Barcott.
The H.J. Anslinger papers
Richard Nixon, a Life in Full
EDIT - THRASH-Sources

Here's a now known quote about Nixon and marijuana ....

“The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar left and black people. You understand what I’m saying? We knew we couldn’t make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did.”

There’s a pretty damning quote from a former Nixon policy advisor in an essay arguing for drug legalization recently published in Harper’s.

The author, Dan Baum, opens the piece with a scene: he finds John Ehrlichman, Nixon’s former domestic policy advisor, working at an engineering firm in Atlanta in 1994. Baum, who is researching drug prohibition politics, starts to ask him “earnest, wonky” questions, before Ehrlichman snaps and gives him this elucidating quote:

“You want to know what this was really all about?” he asked with the bluntness of a man who, after public disgrace and a stretch in federal prison, had little left to protect. “The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar left and black people. You understand what I’m saying? We knew we couldn’t make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did.”
When Baum looked shocked to hear that, he writes. Ehrlichman simply shrugged.

But perhaps he shouldn’t have been shocked. It has long been known that the Nixon administration invented the war on drugs, as Julianne Escobedo Shepard writes at Jezebel, and that the policy has largely failed to help people. Instead, it has incarcerated hundreds of thousands of Americans (mostly black and Latino), and fueled the demand for drugs produced outside the US, most notably in Mexico, where it has fueled horrific violence.

One word in Baum’s quote is particularly telling: “criminalizing.” Here, Ehrlichman admits that drug prohibition was a tool to create criminality in black and leftist communities where there otherwise wouldn’t be.

Other countries have created or considered policies to undo this dynamic. Ireland, for example, decriminalized pot, cocaine, and heroin last year, arguing that addiction is a healthcare issue and not a policy issue. In Mexico, one state governor has suggested legalizing opium in order to give local farmers a legal alternative to illicit opium poppy cultivation.

Baum writes about the Netherlands as an example of a country where drug legalization has worked. And indeed, the Netherlands has been closing jails because it doesn’t have enough criminals. It’s a nice problem to have—and one unlikely to happen anytime soon in the US’s entrenched prison industrial complex.

Source - "We created the war on drugs to criminalize black people"

In 1994, John Ehrlichman, the Watergate co-conspirator, unlocked for me one of the great mysteries of modern American history: How did the United States entangle itself in a policy of drug prohibition that has yielded so much misery and so few good results? Americans have been criminalizing psychoactive substances since San Francisco’s anti-opium law of 1875, but it was Ehrlichman’s boss, Richard Nixon, who declared the first “war on drugs” and set the country on the wildly punitive and counterproductive path it still pursues. I’d tracked Ehrlichman, who had been Nixon’s domestic-policy adviser, to an engineering firm in Atlanta, where he was working on minority recruitment. I barely recognized him. He was much heavier than he’d been at the time of the Watergate scandal two decades earlier, and he wore a mountain-man beard that extended to the middle of his chest.

.... it's too long to post, however, I've linked to Harper's archive, so it should be there for quite a while
... either way, it's a really good, well written read

Harper's Article Mentioned Above - Legalize It All

I am 100% in the legalize it all camp to get rid of black markets & the associated violence.

Outlawing drugs led to other unintended, but easily predictable outcomes beyond just violence too. For example, you create an incentive to smuggle the most concentrated compounds possible, which you can then cut with other substances once you're clear of the highest risk points of smuggling (borders, ports, etc.). Another example is the incentive to synthesize novel, dangerous compounds to try to avoid illegal ones.

I've been all for the legalization of weed for years ....
I don't smoke it (as stated above), but I've always been all for legalizing it ....

I have a number of friends who went from heavy drinkers to heavy marijuana users. They're all clearly self-medicating for anxiety or other issues. They've also all tried the standard list of medications doctors hand out for those conditions.

And they're all clearly 100% better off using marijuana instead of booze. No accidents, fewer arguments, nothing in their house gets destroyed... and also far fewer side effects than they experienced from that standard list of pharmaceuticals.

I think this is generally true of most substances even beyond pot (and have read things about this in the past). People who develop problems with using tend to be self-medicating. Addressing trauma, stress, and mental health generally would result in far fewer addicts than trying to address the issue from the substance side.

Actually, my sister is an even better example:

She was in pain management for years using fentanyl patches. She had very severe migraines since a teenager (severe enough to be hospitalized and put on a morphine drip a couple of times), endometriosis, Rheumatoid arthritis (undiagnosed at the time) , and back issues stemming from a pretty severe car accident.

After Maryland passed medical marijuana, she got a card and has been managing her issues solely with pot.

Hopefully it's working out for her ...
I know quite a few people successfully doing that ...