It is not about using any antibiotic, it is just the tetracycline’s and more specifically tetracycline itself. There are many better grade antibiotics available in 2006 that treat infections.
Tetracycline and anabolic steroids do not go well together. Tetracycline is a broad spectrum antibiotic that has many purposes. It works primarily by inhibiting protein synthesis. Since Tetracycline does exhibit this anti-anabolic effect, it is working in the opposite direction of the anabolic steroids, as well as birth control pills for women (if a women is taking a combined oral contraceptive pill there may be a low risk that this antibiotic may make it less effective at preventing pregnancy). Anabolic steroids increase protein synthesis. Tetracycline may inhibit the functions of the anabolic steroids, or the anabolic steroids may inhibit the effects of the Tetracycline. Additionally, they put additional stress on liver function…
The risk is-if you are using AS, why take a medication that may or could affect the desired results of the medicine you are using, when there are other antibiotic medications available that will not. It may be only a small risk in the overall sense, but not one that I was ever willing to take.
Not being an expert here and it sounds like freakfesrMD is very knowledgeable in the AS subject area, I defer to him for more information.