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 influence of a person's expectations about a drug's effects on their actual experience. While expectations can enhance perceived effects, they do not account for the physiological adaptations that require increased dosages, as seen in tolerance.

The placebo effect occurs when a person experiences real changes in their condition or symptoms due to believing they are receiving treatment, even if it is inactive. This does not relate to the need for higher doses of a drug, as it is not based on the pharmacological action of the drug itself.

Psychological dependence involves a perceived need for a drug to function normally, often characterized by cravings and emotional attachment. While it may lead to increased use, it does not inherently require higher doses to achieve the same effects, as seen in tolerance.

Withdrawal symptoms arise when a person reduces or stops taking a drug they are dependent on, leading to physical and psychological effects. This situation is distinct from tolerance, as it does not imply a need for increasing doses to achieve the original pleasurable effects.