Tachyphylaxis is a medical term describing an decrease in the response to a drug after its administration. Tachyphylaxis can occur both after an initial dose of medication or after an inoculation with a series of small doses. Increasing the dose of the drug may be able to restore the original response. This can sometimes be caused by depletion or marked reduction of the amount of neurotransmitter responsible for creating the drug's effect, or by the depletion of receptors available to which the drug or neurotransmitter can bind. This depletion is caused by the cells reducing the number of receptors in response to their saturation. Some examples include amphetamine, ephedrine, and MDMA which act indirectly through the release of neurotransmitters.
Tachyphylaxis = Needing higher doses to get the same effect.