"The measure of the degree of tolerance that can be shared between different drugs, called cross-tolerance, can be used as an estimate of the similarities of their mechanisms of action. In other words, if A and B are somehow seen by the body as being similar, then a normally effective dose of A will make a next-day's normally effective dose of B weaker than expected. Or not active at all. And B will do the same job on A. If two drugs are different in their ways of doing things in the body, there is most often no cross-tolerance seen. This was described for MDMA and MDA, and is the basis of the argument that they act by distinctly separate mechanisms. A person who used what would be held as an active dose of MDMA for several days lost all response to the drug. He was tolerant to its effects. But an exposure to an effective dose of MDA at the time that tolerance to MDMA was complete, provided a normal response to the MDA. The drugs are not cross-tolerant and the body recognizes them as distinct individuals."
Shulgin, PiHKAL: A Chemical Love Story