I'm a pending pharmacist (1 month!), and have worked in pharmacy for the better part of a decade. I see people acting freaked about waiting a day or two for medications that they have plenty of at home, and this is just day-to-day (no stress of TEOTWAWKI).
Of many medications, other members are right in that pharmacies do not keep stock high enough to fill everyone's at once. If we did, we'd go out of business in a month. However, if you add up the pharmacies in any given city, you'd probably have plenty. Couple that to government stores and manufacturer donations, and you're fine.
Katrina was a good test of TEOTWAWKI, in micro-form. Pharmacies, companies, and pharmacists worked well together to get volunteers down there to straighten out who needs what, who needs it badly, and then getting it to those people. I'm not saying that when the SHTF there wont' be troubles, but there will quickly be organization by pharmacy professionals to make sure people get taken care of. We're not talking about every pharmacy locking up its doors and giving the proverbial finger to the public, quite the opposite.
The rational proof of the above concept is to consider what would happen after the disaster was resolved. Would you keep getting scripts filled at a pharmacy that didn't help you get your meds in an emergency? It would be enough to ruin a company in the aftermath, if they hadn't helped.
But there will be paranoid freaks, and people who use the chaos as an excuse to steal and loot. That's why I find it funny when my peers call me a Rambo for thinking a firearm has a place behind a pharmacy counter. You better believe there are drugged up morons dumb enough to come rob you, they just need a couple of stars to align and an excuse.