There are many types of antibiotics and I personally have taken two kinds (doxycycline and amoxicillin). Yes, usually you take them twice a day for anywhere from a month to 3 months (prolonged used of antibiotics can be quite dangerous), but of course dosage and type of antibiotic depends on your specific case. For me, the amoxi worked far better than the doxy and my derm explained that the amoxi better targets a specific bacteria that is the cause of MY acne. It all really depends on your specific case and sometimes its trial and error and yes they do not always work. You should most def contact a dermatologist for more information.
I've taken Tetralysal for two months now. The good thing about antibiotics is, whilst most people don't stay clear once they come off them, they do get you clear whilst you're on them and you can begin a topical regimen to maintain that clear skin. Probiotics are especially important when you're on antibiotics though. Take the pro in the morning and the anti at night.