The ketamine depression therapy operates with doses of no more than 100mg, taken once or twice a month.
Ketamine abusers typically consume well upwards of a gram, or a couple, daily.
This is a similar situation as the difference between having two glasses of wine on Sundays, and downing a fifth of hard liquor every day.
Edit: FYA: "Matt developed K bladder after taking as much as 15g of the drug a day."
Yeah...if anything, it's testament to the relative safety of the drug, if taking 150x the therapeutic dose daily did not cause the individual to drop dead right away!