maximum tolerated dose
The maximum tolerated dose is the most of a drug a patient can take before the side effects become unacceptable — the highest rung on the ladder before you are forced to climb back down. It is the practical ceiling set not by lack of further effect but by intolerable harm.
MTD is central to oncology, where many cytotoxic drugs follow the logic 'more is better' against the tumour, so treatment is pushed up to the dose just below the one that causes dose-limiting toxicity such as severe myelosuppression. Phase I cancer trials are largely an exercise in finding the MTD by escalating the dose in successive cohorts until toxicity defines the limit.
MTD is patient- and context-specific: it depends on organ function, comorbidities, concurrent drugs, and which toxicity is deemed dose-limiting. It differs from the conceptually nearby maximum effective dose, which is capped by the ceiling effect rather than by harm — for some drugs the two coincide, for others the tolerated dose stops well short of further benefit.
In a phase I trial of a new chemotherapy agent, escalating doses are given to small cohorts until dose-limiting toxicity appears; the dose just below that defines the MTD carried into later trials.
Climb until toxicity stops you, then step back one.
Modern targeted cancer drugs have challenged the MTD paradigm: for them the most effective dose may be well below the maximum tolerated one, so 'optimal biological dose' is increasingly preferred over pushing to MTD.