Read the 2012 Survival Guide linked in my signature--this is straight from there.
As a member of a community built on self-reliance, YOU are responsible for your own basic first aid needs. However, if you or someone you know (or encounter) requires medical assistance, contact an Emergency Services Department volunteer or a Black Rock Ranger, or visit our Medical Clinic near Center Camp at Esplanade & 5:15 or at the Emergency Services Stations behind the Civic Plazas located at 3 & 9 o’clock. Trained emergency medical personnel are on duty 24 hours a day and emergency evacuation is available. The most common problems we encounter are dehydration and lacerations from stepping or tripping on unprotected rebar or tent stakes. Please avoid injury and act responsibly. If you are looking for your friend whom you suspect may have been injured and medi-vac’d from Black Rock City, go to Playa Info for more information.
It's possible Emergency Services may have certain antibiotics (not at 3 & 9 o'clock Plazas though, only at Center Camp where the most serious cases end up). Treatment on the playa is free. If you are very, very sick they'll evacuate you, and THAT will cost you, but them's the breaks. If you are medically stable and they need to write you a prescription they don't have, you'll need to persuade someone to drive you to Reno to fill that prescription. (There is no longer a clinic open in Gerlach.) Take your ticket stub, your wallet (I believe it's $20 to exit/re-enter, plus whatever else you're going to need in Reno) and stop at Gate on the way so you can get a re-entry token.
What do you want for overall health? Well, only you can say for sure. I take children's chewable vitamins and other ordinary supplements, Emergen-C, Pedialyte and Gatorade for electrolytes (others swear by Vitalyte), fruit cups/100% juice/dried fruit, and V-8. And a big first-aid kit. I don't actually wear a mask (I'm more likely to pull a scarf across my lower face) so I can't make a recommendation.