Dear orners3.: Septra DS is a very good alternative to the tetracycline group of antibiotics , used to treat acne. The course is usually between 4-16 weeks, according to the case & the dermatologist's judgement. Topical benzoyl peroxide &/or retinoids can be combined with the sulfa oral medication. Sulfa, has no effect on teeth color.
Oral sulfa drugs are one of the antibiotics commonly used for acne treatment. It can be associated with severe allergic reactions but so are many other drugs. You should ask the dermatologist about other alternatives if you are worried about your daughter been on that antibiotic. Erythromycin is not a very commonly used antibiotic for acne anymore.
Bactrim is an old inexpensive antibiotic that I often use for acne that is resistant to the tetracycline family. It may not be FDA approved, but we use medications "off label" all the time for lots of conditions, including acne. There are lots of options to treat acne including laser and light therapies, as well as hormonal treatments and isotretinoin. Chemical peels and injections help too. You need a great board certified dermatologist to help you control the acne and get it better.