I've only seen this mentioned once in all of the coverage I've read of the descent to madness in New Orleans:
a) What percentage of New Orleans residents were estimated to be hard drug users before the evacuation?
b) What percentage afterwards?
People keep writing about a breakdown in order and so forth, and mention deteriorating mental health as one reason. The other is certainly withdrawal. There were no extra drugs in the city, and I imagine rich dealers fled along with the rest of the middle and upper class of the city.
If 100,000 people were left behind, which was the early official estimate (about 20 percent of 480,000 residents), and just 1,000 of them were drug users--thus estimating the drug usage in the city at 0.2%, laughably low for an urban town--then you have crazy people on the loose hallucinating, with the shakes, with uncontrollable behavior.
Sure, you have the looters laughing as they remove blue jeans and DVD players. Given that about 50,000 people were in two buildings (Superdome and Convention Center), and tens of thousands were in homes and businesses, who was left on the street?
Criminals knowing they could prey on others and steal. Drug addicts without access to drugs (and needing money if they could buy some). Mentally ill people who couldn't get themselves to shelter.
Let's not draw a lesson about society breaking down from this.
Take any city you live in. Pick an area bounded by the old town, the sports stadiums, the convention center, and a dangerous district with high crime and low income. These are often in a direct line or a square or triangle. Now picture everyone with money and cars having left town. Who would be left in your city? The desperately poor, the ill; the deranged, the street people, the criminals; the drug addicts.
The poor and the ill were abandoned in New Orleans. The other groups were lumped in regardless of degree of intent, risk, and sanity.
Society exists in part to protect people who can't protect themselves and in part to protect the majority of society that wants to live in peace from the smaller part of society that is incapable of doing anything but tearing it apart.