If I recall correctly about ketamine, the mind altering effects diminish without sufficient restorative period between dosage. This could further be an issue in two ways. Individuals may associate the effects of ketamine to the mind altering effects only, not realising that it affect the body even if they're not getting the same high ("what I can't perceive can't hurt me"). It could also make individuals try higher dosage more often ("maybe it's bad product, it didn't do anything last night, let's try again").