Is marijuana addictive? As the movement to legalize pot gains traction in multiple states, so does the science suggesting that recreational use of marijuana, now legal in Colorado and Washington, may come at a cost.\u00a0A growing number of researchers have concluded that the drug has addictive qualities. The National Institute on Drug Abuse puts a number on it, saying 9 percent of marijuana users will become addicted, and a first-of-its-kind study published this month in the Journal of Neuroscience concludes that the brains of even recreational pot smokers show changes in key regions associated with addiction. Add to this a growing list of psychiatrists who say they treat marijuana addicts all the time, and you\u2019ve got a fairly good case that\u00a0quitting weed can be a real problem. But last week, a man many consider among the world\u2019s foremost authorities on the subject may have put the debate to rest \u2013 for the Deadheads anyway. Grateful Dead Founder Calls\u00a0Marijuana Addictive \u201cI know guys who are thoroughly addicted to marijuana,\u201d Grateful Dead guitarist Bob Weir told the Huffington Post . \u201cIf they had to stop now, they\u2019d get the shakes, they\u2019d get the sweats -- all that kind of stuff.\u201d All that kind of stuff also includes anxiety, sleep difficulties, decreased appetite, irritability, depression and headaches, says the National Institute on Drug Abuse. Weir, a founding member of a band many say encouraged\u00a0its fans to have mind-bending adventures, is now, decades later, speaking of the perils of drug addiction. It\u2019s no secret that \u201cDeadheads,\u201d many of whom followed the band from gig to gig, regularly consumed copious amounts of drugs.\u00a0In fact, Jerry Garcia, the band\u2019s late leader, openly promoted drugs, particularly LSD, in\u00a0interviews. Nearly 20 years\u00a0after the band broke up, Weir is now speaking of the good friends he\u2019s lost to addiction. \u201cLove yourself first before you go there, because you\u2019re not going to find out anything about yourself to love after going there or from going there,\u201d Weir said. \u201cThis is your psyche, this is your being. It\u2019s going to be influenced, maybe a little bit if you go into it with a light touch, maybe a lot of it\u00a0if you go into it with a heavy hand. \u201cLong experience has taught me that, personally, and, Christ, in the faces that are no longer in my life. It\u2019s a lot of faces. I was good friends with Jimi Hendrix. I was good friends with Janis Joplin. Jerry and I were tighter than that -- and they\u2019re not here anymore.\u201d Of course, Weir was not claiming that Hendrix, Joplin and Garcia died from marijuana addiction, but he firmly disagreed with the popularly held notion that you can\u2019t get hooked on pot. So, now that the Dead have spoken \u2013 or at least Weir has, since he\u00a0says he's not speaking for\u00a0his bandmates \u2013 just what does it mean to be addicted to marijuana? Dependence on marijuana means that one needs to use the drug\u00a0just to feel \u201cnormal.\u201d In order to be diagnosed as marijuana-dependent,\u00a0the user\u00a0must experience at least three of the following in the one year: \tTolerance to the effects of marijuana, meaning that more marijuana is needed to get the same effect. \tWithdrawal from the effects of marijuana, such as irritability, trouble sleeping and depressive symptoms. \tUsing more marijuana than was intended. \tPersistent desire to stop taking marijuana or to cut down and being unsuccessful at this. \tSpending lots of time obtaining, using or recovering from the use of marijuana. \tGiving up important activities in favor of using marijuana. \tUsing marijuana even when it is known that it causes problems. One of today\u2019s biggest names in music says she, too, finds pot addictive. Lady Gaga revealed details of her marijuana addiction, explaining that at one point, she was smoking 15 to 20 marijuana cigarettes a day, according to news reports. "I have been addicted to it and it's ultimately related to anxiety coping and it's a form of self-medication," Gaga said on New Jersey's \u00a0Z100 Morning Show. Marijuana Linked to Heart Problems If\u00a0it\u00a0isn't bad enough\u00a0that\u00a0weed\u00a0is getting negative press from the Grateful Dead's co-founder, a\u00a0new French study says cannabis increases heart problems and may even prove fatal for young and middle-aged users. Researchers in France who looked at 1,979 patients with medical problems related to cannabis use identified 35 serious instances of cardiovascular complications. Twenty heart attacks were recorded, as well as 10 cases involving arteries in the limbs, and three affecting blood vessels in the brain. In roughly a quarter of those cases, the patient died. Most of the patients were male, with an average age of 34. In a statement, lead scientist Dr. Emilie Jouanjus of\u00a0the University of Toulouse said: "The general public thinks marijuana is harmless, but information revealing the potential health dangers of marijuana use needs to be disseminated to the public, policy makers and healthcare providers." \u201cThis unexpected finding deserved to be further analyzed, especially given that the medicinal use of marijuana has become more prevalent, and some governments are legalizing its use,\u201d Jouanjus said. The findings should prompt cardiologists to consider marijuana use a potential cause of cardiovascular disease in patients they see, she said.