BEFORE he suited up to play against the New York Giants in 2012, Jacksonville Jaguars offensive lineman Eben Britton took an ice bath followed by a hot shower. He did his routine stretches. Then he smoked a joint.

In fact, Britton — who also played for the Chicago Bears but left the league in 2014 — tells The Post he played three NFL games while high on marijuana.

“NFL games I played stoned were some of the best games I ever played. Cannabis cements your surroundings,” he says. “A lot of people say they’re useless when they smoke weed. But hell, I played NFL games [while stoned], dude. My performances were solid and I felt really good after.”

Which was the point, since the Brooklyn-born Britton was smoking to relieve, at various times, “psychological distress or sciatica or pain in my shoulders.”

And he was far from alone.

Asked how many NFL players currently smoke pot, Britton estimates “it’s over 50 percent and it could be as high as 75 percent.” A sports agent who asked to remain anonymous says, “The number is rumored to be as high as 80 percent.”

There’s a growing movement among former and current NFL players calling for the league to lift its ban on marijuana. As the pro-football season wraps up Week 2, 18 athletes are sidelined for unspecified substance-abuse violations, which, says sports agent Brian Fettner, who represents a roster of NFL players, “is almost always marijuana.”

Nevertheless, players are willing to take their chances.

Marijuana is so prevalent that one former pro tells The Post, “I had my weed dealer meet me at the players’ parking lot after practice for a delivery.”

SOME health professionals see the benefits of players using marijuana.

Dr. Sue Sisley, M.D., the lead investigator on an FDA-approved cannabis trial study, recounts a recent example of an NFL patient suffering from a rotator-cuff tear that she wound up treating. “Nothing gave him relief — including opioids,” she says. “He was on the bench because he was nonfunctional on the field. Side effects from the medicine had him so sedated that it was literally dangerous for him to play. He was frustrated and lost his position and lost credibility. He tried cannabis and actually got back in the game. He is currently playing now. That is a common scenario.”

Marcel Bonn-Miller, a clinical psychologist serving as the co-principal investigator in a study being done on marijuana and the NFL at the University of Pennsylvania, also sees an upside.

“We think it is a potential alternative to [highly addictive] opiates for pain medication,” he says.

There are reasons why NFLers need to manage pain, of course.

Jake Plummer, who quarterbacked for the Broncos and Cardinals before retiring in 2007, tells The Post he used pot following the hip laparoscopy necessitated by 10 years of taking hits in the NFL. While playing for the Denver Broncos, Nate Jackson used it for the constant aches and pains he endured on the field. Ricky Williams, who retired in 2011 as a running back with the Baltimore Ravens, has talked about smoking up after separating his shoulder during a “Monday Night Football” game with the Dolphins in 2003. Buffalo Bills offensive tackle Seantrel Henderson is currently riding the bench for using marijuana to relieve the pain of Crohn’s disease.

Derrick Morgan, the Tennessee Titans linebacker who says he doesn’t smoke weed but is pushing for the league to explore its possibilities, tells The Post, “Pain, brain trauma, depression, anxiety — those are all things that can be treated well with cannabis.”

Regardless, the NFL maintains its position on marijuana — medical or otherwise. “Medical experts have not recommended making a change or revisiting our collectively-bargained policy and approach [between the NFL and NFL Players Association] related to marijuana, and our position on its use remains consistent with federal law and workplace policies across the country,” states Brian McCarthy, a spokesman for the NFL.

Over the past two decades, the game of pro football has become more physically demanding and dauntingly stressful on the bodies of players. Lumbering pros in the slovenly mold of the Bears’ 335-pound William “The Refrigerator” Perry are now few and far between.

“The toll on the body is tremendously greater than it had once been,” says Dr. Nelson Vetanze, who previously served as team chiropractor for the Denver Broncos and has treated some 800 NFL players over the past 44 years. “Players are bigger, stronger and faster. They hit harder, and [bone] joints get targeted.”

But players admit that the line between smoking for fun and smoking for therapy is blurred.

“Right after practice, me and one to three other [players] would go to somebody’s house [to get high],” Britton recalls. “Smoking helped me to socialize with teammates. We let our guards down . . . But a lot of the time [stoned talk] wasn’t even about football. Cannabis took us out of football. We were hanging out and just being people. It brought us closer together as teammates.”

‘Right after practice, me and one to three other [players] would go to somebody’s house [to get high].’ - Eben Britton

There’s also the issue of whether or not abusing a substance improves performance — one of the main reasons to ban it in the first place.

Like Britton, Nate Jackson — who retired in 2009 — believes that cannabis doesn’t just ease pain, it can also give players a leg up. “I became a better athlete from it,” says Jackson. “Marijuana improved my hand/eye coordination. Being high on cannabis allowed me to see the game on a different level. It made me a more creative player.”

Jackson, whose memoir “Fantasy Man” comes out Tuesday, recalls how, on his off days, he recuperated from the previous game’s bruised muscles and banged-up bones by relaxing and getting stoned.

After one particularly brutal game that ended with a trip to the emergency room for a rib injury, “I could not breathe real deeply without pain,” he says. “But I stayed in bed, smoked marijuana, and healed in just a couple of days.”

It also kept him off of joint-damaging anti-inflammatories and addictive opioids such as OxyContin — which is “eaten like candy” in the NFL, Jackson says. “Players get handed bottles of pills.” The Titans’ Morgan says he’s heard of footballers becoming dependent on the drug.

“The dreaded end result [of being prescribed an opioid] is needing 400 milligrams a day just to feel normal,” says Dr. Sisley. “I’ve seen professional athletes graduate to heroin because they can’t keep getting OxyContin.”

On the other hand, she adds, “Many players have been able to get off opioids completely by using cannabis as monotherapy.”

Health officials maintain, though, that marijuana comes with potential drawbacks. According to the National Institute on Drug Abuse, recent data suggest that 30 percent of marijuana users have some degree of “marijuana use disorder.” This often aligns with a dependence on marijuana that, in some cases, can become an addiction. The institute also reports that after inhaling marijuana, a user’s heart rate may increase by 20 to 50 beats per minute.

Buffalo player Seantrel Henderson is suspended after failing a urine test. Though a repeat offender, Henderson maintains that this time he had reason for using: Pot is the only thing that provides relief from the pain of Crohn’s disease, a malady for which he underwent surgery earlier this year.

“It’s unfair punishing him over the Crohn’s,” says Fettner, his agent. “[The NFL] should drop the testing for marijuana. They don’t test for nicotine. They don’t test for alcohol” — which some players abuse in lieu of weed.

But booze isn’t a viable answer for pain relief; it brings on hangovers, liver damage and weight gain.

“Alcohol is a central-nervous-system depressant, so it makes players sluggish and leads to poor performance on the field,” says Sisley. When consumed in excess, regular boozing can lead to “alcohol impairment, brain atrophy and domestic violence. Marijuana does not cause any of those things.”

The trick with marijuana, though, is getting around the league’s annual urinalysis (administered up to 10 times per month for players, like Henderson, who’ve failed a first time). The process became increasingly invasive after 2005, when Minnesota Vikings running back Onterrio Smith got caught by airport security possessing a Whizzinator — a kit that includes a hollow prosthetic penis — and vials of white powder that he said was dried urine.

He claimed to be bringing the rig to his cousin, but it led to the NFL taking measures that prevent players from employing similar devices to fool a urine test.

As a result, says Jackson, the test is now conducted under close scrutiny. “You strip down [so you are exposed from] your knees to your neck,” says Jackson. “A guy stands right next to you, stares at your penis, and watches you pee. Having an old man with a mustache staring at you does not make you want to pee. You have four hours to do it. If you can’t, you fail the test.”

As for the future of marijuana in the NFL, it’s as cloudy as Britton’s living room during those post-practice smoke-outs.

“There is a police-state mentality in the NFL,” says Jackson.

A retired coach who worked in a state where marijuana is now legal agrees: “As far as the NFL goes, [legalizing pot] is a nonstarter.”

While the league remains intractable, Fettner the sports agent believes that a progressive view will benefit all parties. “NFL testing for marijuana is not doing anybody any favors — including the NFL,” the agent sums up. “It makes them look bad, it makes the players look bad, and it glamorizes marijuana, which is the exact opposite of what you want to be doing. Marijuana has become less of a big deal for everyone — except for the guys who make the rules.”