What Is a Drug?
Most people would agree that heroin is a drug. It is a white powder that produces striking changes in the body and mind in tiny doses. But is sugar a drug? Sugar is also a white powder that strongly affects the body, and some experts say it affects mental function and mood as well. Like heroin, it can be addicting. How about chocolate? Most people think of it as a food or flavor, but it contains a chemical related to caffeine, is a stimulant, and can also be addicting. Is salt a drug? Many people think they cannot live without it, and it has dramatic effects on the body. A common definition of the word drug is any substance that in small amounts produces significant changes in the body, mind, or both. This definition does not clearly distinguish drugs from some foods. The difference between a drug and a poison is also unclear. All drugs become poisons in high enough doses, and many poisons are useful drugs in low enough doses. Is alcohol a food, a drug, or a poison? The body can burn it as a fuel, just like sugar or starch, but it causes intoxication and can kill in overdose. Many people who drink alcohol crusade against drug abuse, never acknowledging that they themselves are involved with a powerful drug. In the same way, many cigarette addicts have no idea that tobacco is a very strong drug, and few people who drink coffee realize the true nature of that beverage.
Well, since Islam doesn't allow drinking alcohol for the sake of protecting the body from harm (Noble Verse 2:219)"They ask you about intoxicants and games of chance. Say: In both of them there is a great sin and means of profit for men, and THEIR SIN IS GREATER THAN THEIR PROFIT. And they ask you as to what they should spend. Say: What you can spare. Thus does Allah make clear to you the communications, that you may ponder." then it is quite simple and obvious that it would prohibit use of harmful drugs such as weed and pot.
Just because drugs were not known 1400 years ago to mankind, and were not mentioned in the Noble Quran, it doesn't make it in anyway lawful to use without a doctor's prescription such as in the case of marijuana.
What Is a Drug?