I'm 30, male, and have been taking 50mg doxy twice a day for 3 years. I have tried everything. Doxy works VERY well for me. Nothing else does or has. After this long, I have little or no acne at all, but, if I stop taking doxy, after 4-5 days I can feel it coming back. If I continue to not take it it comes back... Doctors tell me to keep taking it, even after three years, they say eventually the internal infection will dissapear. Here's bloody hoping it's soon!! My diet is very healthy, drink litres of water a day etc. Have had acne since mid teens and this last three years have shown the best results ever. And no, I havnt become immune, have taken the same dose since I began in 2006. Each to their own I guess.