Pretty much that. The drug would have to be some sort of key to unlock access to the alien consciousness. The key (drug) is definitely too small to encode such a complex mechanism, so the logic would have to be with the thing the key fits into, the way it interacts with our brain. But when you consider that it would have to remain stable over millions of years of evolutionary changes, or be redundantly specified strongly enough to survive such genetic drift... it really stops making any kind of sense from an information theory standpoint.

From a physical standpoint, it's just not possible that our minds could communicate with an alien consciousness when you consider the effects of speed of light on communication latency, and the speed of the nervous system. We have a hard enough time preventing VR headsets from making the user nauseous from latency, to say nothing of communicating with some sort of non-local alien entity.

If you're willing to throw all that out.. well then we're in pure fantasy land.