Final answer:
Long-term drug use is likely to increase the reward threshold, where the user needs to consume more of the drug over time to experience the same effects, due to tolerance.
Step-by-step explanation:
The effect of long-term drug use on reward threshold is typically an increase. This means that over time, the person using the drug will require more and more of the substance to achieve the same feeling, or 'high', that they experienced with a smaller dose previously. This phenomenon occurs due to a process known as tolerance, which refers to the body's adaptation to a drug's effects, leading to diminished response to the same dosage.