A drug is any substance, other than food that alters a person's mind or their body. You notice that it doesn't say any unnatural substance or any synthetic substance. Most of those drugs are from plant sources. Even aspirin came originally from the bark of willow trees. And the base needed to manufacture heroin is opium gum which comes from the opium poppy. So yes, Marijuana is absolutely a drug.
Regular marijuana smokers commonly experience cravings to smoke weed. These cravings can overcome their better judgment, and can cause the user to smoke even though he or she knows there are negative consequences involved.
That is another indication of addiction: continuing to repeat some behavior despite negative consequences.
Another sign of addiction is requiring larger and larger doses of the addictive substance in order to experience the same effects. This is also well documented with the use of marijuana. Larger amounts of the drug or more powerful strains are needed to keep the user satisfied and to keep experiencing similar levels effects.
Many regular marijuana smokers find that they are unable to stop the use of the drug on their own, so much so, that marijuana is the most common illegal drug that sends people to addiction treatment in the US.
When regular pot smokers try to quit marijuana, they commonly experience irritability, depression and cravings. These are all withdrawal symptoms and are further indications that smoking weed can cause addiction.
So it really is no longer in question, whether or not the regular use of marijuana is capable of addicting a person, but there is one more very important aspect to this: The likelihood of addiction goes up, the earlier regular use begins. This is true with any addictive drug. The chances of becoming an addicted smoker or drinker go down, the later first use occurs.