This is not my area of epidemiology, so take this with a grain of salt, but my best guess is that the impact is negligible, for the following reasons
(1) Mutations that confer resistance are specific for classes of antibiotics. A mutation that provides resistance to Doxycycline will not help the bug against Penicillins.
(2) If I understand correctly, the reason tetracyclines are approved for acne is that they are no longer the first line choice for empirical treatment of any serious infections. This is because tetracycline resistance is already widespread. In other words, the FDA have allowed Doxycycline to be used with this indication specifically because they are not worried about increasing the prevalence of resistance.
(3) If you are taking these drugs while you are healthy, there are no pathogens that are subjected to adaptive pressure by your use. Of course, you will have commensals that are subjected to this evolutionary pressure, but I think the risk that these bacteria acquire pathogenicity and escape to start an epidemic is minimal.
I also want to note that, based on anecdotal experience and conversations with dermatologists, my impression is that long term treatment with doxycycline is probably not the most effective way to treat acne. By far the most effective treatments are variations of retinoic acid. If you have serious ongoing acne, Isotretonoin (Accutane) is the only thing that will really help. If you have less serious acne or want maintenance treatment after taking a course of Accutane, topical medications which contain variations of the same active ingredients (retinoids) are very effective. Your alternatives are creams that only include retinoids (Such as Adapelene), which tend to be cheap; or you can take a cream that also benzoyl peroxide (such as Epiduo). The second option is more effective, but also more expensive, and if you get it on your clothes they may get bleached from the peroxide.
This is not my area of epidemiology, so take this with a grain of salt, but my best guess is that the impact is negligible, for the following reasons
(1) Mutations that confer resistance are specific for classes of antibiotics. A mutation that provides resistance to Doxycycline will not help the bug against Penicillins.
(2) If I understand correctly, the reason tetracyclines are approved for acne is that they are no longer the first line choice for empirical treatment of any serious infections. This is because tetracycline resistance is already widespread. In other words, the FDA have allowed Doxycycline to be used with this indication specifically because they are not worried about increasing the prevalence of resistance.
(3) If you are taking these drugs while you are healthy, there are no pathogens that are subjected to adaptive pressure by your use. Of course, you will have commensals that are subjected to this evolutionary pressure, but I think the risk that these bacteria acquire pathogenicity and escape to start an epidemic is minimal.
I also want to note that, based on anecdotal experience and conversations with dermatologists, my impression is that long term treatment with doxycycline is probably not the most effective way to treat acne. By far the most effective treatments are variations of retinoic acid. If you have serious ongoing acne, Isotretonoin (Accutane) is the only thing that will really help. If you have less serious acne or want maintenance treatment after taking a course of Accutane, topical medications which contain variations of the same active ingredients (retinoids) are very effective. Your alternatives are creams that only include retinoids (Such as Adapelene), which tend to be cheap; or you can take a cream that also benzoyl peroxide (such as Epiduo). The second option is more effective, but also more expensive, and if you get it on your clothes they may get bleached from the peroxide.