Hi, C.
I have plenty of experience with doxycyline. I was on it for four months, until I couldnt tolerate it anymore b/c of the sun. I started off with 200mg, then increased to 300mg and then 400mg per day. Depending on how severe your symptoms are, it is ok to start off slow. I was extremely sick and was herxing on only 200mg a day. My Dr started me off slow, and waited until she knew I could handle a higher dose. If you have severe symptoms, then I would recommend starting off on a lower dose than I did.
Most people have issues with the sun and doxy - I couldn't even drive without the sun burning my hands! And I have 3 kids, so I knew I couldnt handle the beach this summer if I stayed on it. I kind of plataued anyway, so in the end it was a good thing that i switched from doxy to bactrim ds.
you are taking magnesium bc lyme depletes magnesium from your muscles, and bc of this you end up having twitches and muscle fatigue. If you dont have this already, then the magnesium will hopefully prevent this from happening. I still have the twitches, and vitamin B12 injections are now helping. But I am still on magnesium 800mg per day.
tickbite is right about the cyst form of lyme - are you on any herbs?
please let us know how you are doing!!