Opiate agonists like oxycodone, morphine, or heroin will have an increasing opiate effect as the dose is increased. A graph of dose vs effect for agonists yield a straight [diagonal] line.
This is not the case with Suboxone. With Suboxone the line would start out diagonal but then level off and become horizontal, as increasing doses would not increase the opiate effect. Again, this is the basis for the use of Suboxone for opiate addiction treatment. The horizontal portion of the curve would have an opiate effect equal to 30 mg of methadone, meaning that a person taking Suboxone would not be able to get a greater effect from Suboxone or other forms of buprenorphine. For most people, if Suboxone is taken correctly the flat part of the response occurs at about 2-4 mg of the drug.
The ceiling effect is also the reason for precipitated withdrawal; if a person is taking an opiate agonist at a dose and effect greater than 30 mg of methadone, taking Suboxone will force the opiate effect lower, causing withdrawal symptoms.(source