What occurs when a larger amount of a drug is required to achieve the same effect that a smaller amount once produced?

The situation described—where a larger amount of a drug is required to achieve the same effect that a smaller amount once produced—is known as tolerance. Tolerance develops when the body becomes accustomed to a particular substance, resulting in a decreased response over time to the same dose. This often happens with medications that are taken frequently or over prolonged periods, as the body's biological systems adapt to the presence of the drug, necessitating a higher dose to elicit the same therapeutic outcomes.

In contrast, the other options do not apply to this scenario. Synergism refers to the interaction of two or more substances that produce a combined effect greater than the sum of their individual effects, rather than an increased dose requirement. Trade name pertains to the commercial branding of a medication rather than physiological changes in drug effectiveness. O.D., or overdose, implies taking a harmful amount of a drug, which does not specifically address the gradual need for an increased dosage to achieve a desired effect. Therefore, tolerance is the only term that accurately reflects the phenomenon described in the question.