The sedative would only get you three layers deeper. If the sedative is used when already in a dream state, then you could go four levels down, although the person using the sedative would only think they were three deep. The sound of the top falling over, which is pretty much accepted as canon, is all the proof I need. Although for all the diehard 'Mol is right' theorists out there, since there's no image, we don't know Cobb didn't knock it over himself. At that point, wouldn't you?Omega V said:Actually, I can prove beyond a shadow of a doubt the entire thing most certainly is NOT a dream:as I understand it, it is explicitly mentioned that the maximum number of dream layers that can exist simultaneously is limited by the quality of the sedative used. The very best sedative in the world can produce three layers+ limbo. Assuming this is true, since all three layers+ limbo are seen towards the end of the movie it almost completely rules out the possibily of Cobb still being in a dream world. For example, if Cobb had been if the dream world since he was in limbo with his wife, then it would require at least 5 layers+ limbo, well beyond the capabilities of any sedative.( of course he could of just dreamed that the sedative has any impact on the maximum number of dream layers, however, there is no evidence that the dream world can trick someone into believing something that he would know to be a blatant falsification )
If Cobb had been dreaming since the scene where he meets the chemist, then it would still require at least 4 layers+ limbo, still beyond the capabilities of the sedative.
Also, the top is clearly seen loosing momentum at the very end.
Now that I think about it, this interpretation of the ending is actually very similar to Shutter Island's ending.
Off-Topic, Ken Watanabe is the man.