In order to get pleasurable effects from the drug he has been taking for a while, Theo finds that he must take a higher dose. This is likely because Theo is experiencing

A drug expectancy effect refers to the anticipation of experiencing certain effects based on prior experiences with a drug. While this can influence how one feels after taking a drug, it does not explain the need for increased dosages, as the effect is about perceived rather than actual pharmacological changes.

The placebo effect occurs when a person experiences a perceived improvement in condition due to their belief in the efficacy of a treatment, despite it having no therapeutic effect. This effect does not account for the necessity of higher doses, as it is not based on actual drug efficacy or tolerance.

Psychological dependence involves emotional or mental reliance on a drug, often characterized by a craving for its effects. While this may lead to increased usage, it does not specifically explain the need to increase dosage to maintain the same effects, which is primarily a function of tolerance.

Withdrawal symptoms arise when a person reduces or stops using a drug they have become dependent on, leading to physical and psychological symptoms. Theo's situation does not indicate withdrawal, but rather an adaptation to the drug's effects that requires a higher dose to achieve the same level of pleasure.