There is no such thing as a 100% corn free antibiotic. BUT, if you have an infection and need a safe-ish antibiotic right now, here are some possible options:
Rocephin (Ceftriaxone) Injection
Many corn allergic folks have had success with a rocephin injection. I have never personally done this, but here is the package insert. It looks like it can be prepared in water or in ethanol. Ethanol is corn alcohol, so request that the solution be in water, and double-check the package insert in the office to be sure that the inactive ingredients look safe. (Cross reference with the corn allergens list.)
Additionally the rocephin injection is often combined with a numbing agent, lidocaine, which can be skipped. Preservative free lidocaine (Xylocaine is one brand) can be corn free but you will need to check package inserts, not all clinics have the corn free version in stock.
There may also be other injections that could be safe. Check using these resources: how to find inactive ingredients of medications.
Update 2015: I personally have gotten this injection… There are a few different brands but basically I got a Ceftriaxone injection that was a powder with only the active ingredient. My doc mixed it only with distilled water. It hurt a LOT. I am a grown up with a fairly high pain tolerance, and I was crying a little. I did have a mild/moderate reaction from it that passed within 3 hours, and I believe it was a corn reaction from the growth medium, but I don’t really know. I recommend pre-treating with any safe antihistamines you have before getting the injection.
The Zithromax brand, 600 & 250mg, are very corn lite. Not corn free, but I’d take them in a pinch. The generics all seem to have corn starch, so brand name only.
There are many corn-lite formulations of cephalexin.
Here’s a list of all the formulations of cephalexin and their inactive ingredients. Depending on sensitivity, you may be able to get away with taking one that just has as few ingredients as possible and no corn starch.
If you need pills and can’t tolerate potentially corny derivatives, you will need to have your antibiotics compounded. Here’s some good advice on how to do that.
Note that much like probiotics, antibiotics are not 100% corn free just due to what they are. They are a product of microbes and are almost always grown on a medium containing corn sugar.
So with that in mind, I would avoid antibiotics as much as possible, opting for natural remedies as much as you possibly can. But sometimes you have no other choice, and when that’s the case, be aware that even if you get the “cleanest” antibiotic you can get your hands on, you will still be getting some corn, and prepare yourself accordingly.