Seen from a motion picture
One of the reasons that we know that Medical Cannabis was well in use before written history is because so many civilizations (as soon as they obtain a written language), speak of it in their writings, in the past tense, as if it had already been in use for many hundreds of years before then.
It was the Sumerians (southern Mesopotamia or Iraq) that seem to be the first ones to have developed a practical form of writing that today is known as Cuneiform at or around 3,000 BC. And while pictographs (using pictures to portray whole words or concepts) were in use long before that, Cuneiform writing provided a means of recording specifics. (thus it can be said that written history began at that time) However, it should be noted that archeologists do disagree as to the exact meaning of specific words.
For example: All agreed that one of the earliest known Medico-texts can be found on Sumerian clay tablets, probably written around 3,000 BC. But there is a great deal of disagreement as to whether Cannabis itself is actually mentioned in these early Sumerian texts or not. This is because, unfortunately, the ancient Sumerians did not speak English and so did not write down the actual words, "Medical Marihuana." Instead what they did write down were strange medical terms, (hypothetically speaking) like, "the Lilly of the white lake", "the Bark of the black tree" etc., medical terminology that today is (well), quite subject to interpretation.
However, the chances are very high that Cannabis indeed was mentioned and used in their medicines. After all, how many medicines are there out there that can also be "spun" into cloth, or be used in rope making?
According to R. Campbell Thompson in his 1936 "A Dictionary of Assyrian Chemistry and Geology," the following Sumerian words stood for either parts or the whole of the Medical Marihuana Plant:
In addition Thompson goes on to point out various sub-meanings of the words; i.e. [In A-ZAL-LA the 'ZAL' part means to "SPIN," such as in, the plant used in spinning etc.] As well as the similarity of these words and with those making reference to Cannabis in other languages.
But again, the exact meaning of these words is still somewhat open to interpretation, and the argument can be made that all of these references refer only to (what is now known as) Industrial Hemp and not Medical Cannabis per say. One must than ask the question; Why would references to ropes or cloth, be made mention of in what are otherwise medical texts?
Few however deny that by the time that the Assyrian Empire was in existence, the use of Medical Cannabis was well established. Thus, the argument is not if the Mesopotamians made use of Medical Cannabis, only during what time-period did its use begin.
Without Question  the ancient Assyrians (today central Iraq) made use of Medical Cannabis and it is specifically mentioned on clay tablets found at the library of Assurbanipal, an Assyrian ruler, who lived around 650 BC. It along with another 400 other drugs comprised the whole of the Assyrian pharmacopoeia of its day.
It should be noted that Assubanipal's medical library (maybe the world's first such library) consisted of tablets that went back as far as 2000 BC.
About the word:
Just like in the English Language where we have many words like Hemp, Cannabis, Marihuana etc., to describe one plant, so to did the ancient Assyrians. R. Campbell Thompson in his 1936 "A Dictionary of Assyrian Chemistry and Geology," gives the two following Assyrian words:
AZALLUU (A-ZAL-LU-U)But by far, the word, or sound pattern, that causes the most controversy as well as the most interest is: "Quunabu."
According to Jurgen Thorwald's book "Science and Secrets of Early Medicine" pp.170:
"Quunabu - such was the Assyrian name for Indian hemp. This is basically the same word as it was later known by cannabis (Cannabis India), and hemp is cognate with it. . . . it was often employed in Mesopotamia to relieve the pain of bronchitis, bladder trouble, rheumatism, and as a remedy for insomnia."And here it should be pointed out that we obtain the word Cannabis from the Romans who obtained it from the Greek word Kannabis, who in turn could have, and probably did, obtained it from the Assyrians. This would (if correct) make it one of the oldest words known to men. 
However, it should be pointed out that the sound pattern, "Quunabu" is just one interpretation of the cuneiform symbols. For example, these same cuneiform symbols can also be interpreted as sounding more like “Qunapu” than “Quunabu” (or for that matter a whole host of other but similar sounds patterns) and for all we know, any one of them could be the correct pronunciation.
But in any case few if any Mesopotamian scholars dispute the fact that Medical Cannabis WAS in use by the time of the Assyrian Empire. The only dispute is in reference to exactly when this medicine first come into use. Most (but not all) however, attribute it to before 3,000 BC when the cuneiform form (of wedge shaped writing) was first developed.
 Actually Clay tablets.
 Without question, is defined here as being the vast amount of modern day archeologists.
 It is interesting to note that Jurgen Thorwald is one of the few who does not believe that Cannabis is was in use by the Ancient Egyptians.
 Our apologizes to female type people, the English language just doesn't work well some times.