QUESTIONS ABOUT DOXYCYCLINE

i've been using doxycycline for almost a year and a half for my acne and it has started to lose its effect on treating my acne.  My friend has told me about minocycline and that it has really work well for him and i was wondering if the change in medication would help?  I went to see a pharmacist today and he told me that 90% of doxycycline and minocycline are the same.  Also, i wanted some opinion on GERD and barrnetts diesase.  During the prolong period i had taken doxycycline, there would be theses days where i thought i would have taken the medication wrong and it had gotten stuck in my throat and released.  I would have the pain in my throat all day, that would really bother me, and this would happen i would say at least twice a month, do i need to be worried about anything????  Also, are there any side effects or things i need to be worried about for prolonged use of doxycycline?? I have been using doxycycline for a year and a half, as i stated before and had a purchased a 500 pill supply about 6 months ago, and i was wondering if it is possible for doxycycline to lose its effectiveness or anything else over time (is there a time frame of expiration)?  I know minocycline may become poisonous over time, so it that the same for doxycycline?  Are there any other oral medications that you would recommend that i can talk to my doctor about taking?  I had taken accutane about 2 years ago and took it for about 3 months.  First month, it made me break out all over my face (like i have never broke out before in my life) and after that it was okay for a couple of months then the acne came back.  Please help me, any infomation would be appreicated.  THank you