Notrightquiet is perfectly right. While "drug" is a rather loosely used term (I prefer 'psychoactive'), cannabis fits the definition because it contains chemicals that directly affect the central nervous system and may cause significant changes in the way you think. Many people including myself have developed a tolerance to marijuana's effects and find it easy to go about ones normal everyday activities whilst stoned. I've also used heroin on occasion and must say that it also allowed me to perform my daily tasks as I normally would have, perhaps even better. Is heroin not a drug now?

I believe cannabis is illegal because it is a threat to the tobacco, alcohol and pharmaceutical industries not to mention oil and lumber. For those interested in how it actually became illegal read a few chapters of The Emperor Wears No Clothes.