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The reason the physical body, and the feelings of pain, and other "abnormal" feelings, in particular; are not felt and sensed by the central control unit of the body (i.e. the brain) under the influence of certain drugs, is the fact that the balance, or whatever level considered to be the normal level, of one (and often two, or three at the most) certain chemicals and substances inside the body is pushed off the edge (in one way or another) through of a very broad distribution of the after-effects of the change in processes by which it is achieved (which could be a change in the initial amounts of the chemicals present or a change in the form of the process), which is propagated throughout the whole body in an amazingly efficient and fast manner.

More On The Nature of These Processes:

They are time-variant, and changing, in both increasing or decreasing directions (obviously more towards the decreasing one in the case of the particular feelings mentioned above). They can be modelled by first, second or third order, at the most, differential equations.

The level of the each and every chemical determines the current state of the brain (many of these levels can be ignored since they do not change throughout these processes, so their rate of change is zero). The changes in the levels alter the current state of the brain in their own distinct ways, which are obviously different in any individual, but similar in groups of individuals.
The details of the propagation can be traced down to the smallest scales since it might be possible to model their natures in different ways on different scales, which adds to the complexity of the whole process.

The infinitely large surface area covered by the micro- and nano-level veins and pathways throughout the whole body makes this all possible.