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.
Zithromax Brand
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.
Cephalexin
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.
Corn Allergy Girl 
Does anyone know anything about Bactrim? It’s said sodium starch which could be potato, wheat or corn. I need to take this antibiotic but not sure if I should. THANK YOU VERY MUCH!
Hey Jeanne. You should take urgent questions to this group. I am just one person and don’t always look at or respond to comments. https://www.facebook.com/groups/cornallergy
The generic of Bactrim is Sulfamethoxazole / Trimethoprim. I looked up through the first dozen or so pages of results on DailyMed following the backup method described here: https://cornallergygirl.com/2013/03/26/finding-inactive-ingredients-of-medications/
Unfortunately the instructions on the first part of the post are referencing a service that has been taken down, but the second half still works.
Every tablet version of the drug I found had both corn and potato starch.
Since I react to the antibiotics themselves due to being fermented on corn, even when they don’t contain additional corn, I would not take a tablet with corn starch. Depending on the reason for needing it I would:
1) call my compounding pharmacist and see how fast they could make a version for me with at least no corny inactive ingredients. https://cornallergygirl.com/2013/10/31/ask-corn-allergy-girl-finding-a-compounding-pharmacy/
2) talk to my doctor about an alternative medication such as the ones mentioned above.
3) Even if the rocephin or zpack mentioned above weren’t appropriate, I would use dailymed to find some brands of alternative drugs that have less corn in them. I would then have my prescription transferred to basically every pharmacy/pharmacy chain in my area one at a time and then call them and ask them what brand of the drug they carry. Every chain gets just one brand of the many available usually, and they can’t look that up in their system without a script on file so that’s why I would do it that way. This could take a while so finding out from the group what has worked for people, or getting it compounded would be preferable.
4) I would definitely be taking the max of all of my preventative and rescue medications while taking the med to minimize reactions. Having antihistamines on board for a couple hours before a trigger is far more effective than taking them after the fact. My protocol is discussed a few places but here is one: https://cornallergygirl.com/2017/11/25/hemolytic-anemia-g6pd-deficiency-how-i-survived-intravenous-corn-derivatives/