ceiling effect
Pressing harder on a gas pedal that is already floored does nothing. The ceiling effect is that point in drug action where you have hit maximum output: once the curve flattens at its top, giving more drug buys no extra benefit, only extra risk of side effects.
Mechanistically, the ceiling can arise because all the relevant receptors are occupied, because a downstream step is saturated, or because the drug is a partial agonist whose intrinsic activity caps the response below the system's true maximum. In each case the dose–response curve has a true plateau, the Emax, beyond which higher doses do not raise the effect.
Clinically the ceiling effect is double-edged. For non-opioid analgesics like ibuprofen or paracetamol it is a safety feature explaining why exceeding the recommended dose adds toxicity without more pain relief; for partial-agonist opioids like buprenorphine the respiratory-depression ceiling improves the safety profile. The practical lesson is the same: above the ceiling, escalate the drug and you escalate only the harms.
Loop diuretics like furosemide have a high ceiling and keep working at large doses; thiazides have a low ceiling, so doubling the dose past it adds side effects but little extra diuresis.
High-ceiling vs low-ceiling drugs.
Don't confuse a ceiling effect with tolerance. A ceiling is reached at one moment because the system is saturated; tolerance is a fading response over time despite the same dose.