Red leg is a symptom, so you have to figure out what it showing you. Often, the bacterial infection that causes what was classically called "red leg", may be caused by more than 20 bacterial agents and frequently doesn't even present with the classic symptoms of hemorraging on the ventral surface around the legs,). Now a vet may prescribe an antibiotic while conducting tests to determine the actual disease as some of the septicemias can proceed to death of the amphibian within hours of presentation of the symptoms.
The vets I work with have used a number of injectable antibiotics with great results. They are using some of the newer latest generation antibiotics with great effect. The drugs used have been combination such as amakacin and pipercillian or some of the newer ones (which I can't remember off the top of my head) but pretty much if your buying it over the counter, you are very unlikely to be making a difference and can potentially harming the animal.
Some more comments,
Ed