That’s an excellent question. There are no exact statistics to say the eact number of people addicted to cannabis. Part of the issue is the definition of addiction and dependance as well as what some people call "marijuana use disorder". As a relatively new term, there have been an very rough estimate that 4 million people suffered from marijuana use disorder in 2015. This is different than an addiction which would be when a person cannot stop using a drug even when it interferes with many aspects of their life. There have been some studies that suggest that 9% of people who use cannabis become dependent on it. So the term "marijuana addict" is really a misnomer.
People who use marijuana frequently and with a high amount a THC, often report irritability, mood and sleep difficulties, decreased appetite, cravings, restlessness, and/or various forms of physical discomfort that peak within the first week after quitting and last up to 2 weeks. Marijuana dependence occurs when the brain adapts to large amounts of the drug by reducing production of and sensitivity to its own endocannabinoid neurotransmitters.
It’s estimated that 35% of tobacco users and 15% of alcohol users become addicted to those products. However, there are people who have a susceptibility to many different products. You can even find people who "need" Coca-Cola.
Perry Solomon, MD
Dr. Solomon’s answer is measured and accurate, but the issue of what you mean by "addiction" is critical. While cannabis has not been extensively studied in many areas this is actually one where there is much published science. However, changes in the use of the term "addiction" have blurred the lines. If by "addiction" you mean the kind of situation created by regular use of opioids, alcohol, benzodiazepines, etc., with developing tolerance, dose escalation, and a withdrawal syndrome, the answer is: there is NO physiologically significant syndrome occurring after withdrawal from regular marijuana use. Minimal changes in sleep and dreaming are regularly reported, and the impact on mood and behavior appears similar to what you would expect when withdrawing anything that has become an established influence via regular use eg. coffee, running, etc. The typical manifestations of addiction in which need for the drug effect becomes the dominant driver of behavior is hardly, if ever, seen with cannabis. The prohibitionist wing of the medical establishment has redifined addiction to cover what used to be termed "habituation," so I find the current scientific literature in this area trivial. "Cannabis use disorder" appears to me to be defined as using more cannabis than your doctor thinks you should. If you ignore your responsibilities to focus on getting and using cannabis you have a disorder, but the cannabis use isn’t the cause of your disorder, it’s the manifestation.