Illegal drugs are not sold in the free market and as a result the sold good is subject to the whims of the local outlaw manufacturer or distributor. As a result, they often have reduced quality, which can cause a lot of unnecessary health side-effects on their users, including many deaths.
During the prohibition on alcohol it was observed that the quality of alcoholic beverages has heavily decreased, and the same thing happens now with the currently illegal drugs.
Since it is hard to find high quality drugs, when drugs of a high quality are found the drug user will buy as much of it as they can afford, especially if it's a good deal and therefore they will have a much larger quantity around than they normally would have and therefore tend to consume more drugs.
Imagine that liquor fluctuated in price and quality. You go to the store and sometimes there are harmful chemicals in the liquor, sometimes the prices are extremely high, sometimes there is no liquor at the store at all. Only once in a blue moon are prices very good and quality good (say, it only happens once a year). In that case, you would buy as much as you could afford. Naturally, people who only use alcohol might not agree with this, but that's because they have never been in that situation, since alcoholic beverages are fully legal.