Ceiling effect of a drug

(page 6, Pharmacology: An Introduction, 5th Edition, by Henry Hitner and Barbara Nagle) "Eventually a maximal response [of a drug] is usually obtained; further increases in dose do not produce any greater effect. This point is known as the ceiling effect. The ceiling effect reflects the limit of some drug classes to product a particular effect. Above a certain dosage no further increase in effect is observed. Doses above those needed to produce the ceiling effect usually cause other undesired, often toxic, drug effects. Drugs within a drug class that are more potent than other drugs in the same class will produce the ceiling effect at the lower dosage, but the will not 'raise the ceiling'. Drugs that continue to cause an increased effect as long as the dose is increased do not have a ceiling effect."