100mg a day probably won't do much. The drug could possibly reach a high enough blood level to slow the growth of new bacteria (bacteriostatic), but won't get high enough to actually kill the ones that are already there (bactericidal).
A 200 mg dose can be effective if you catch the infection early, but most LLMDs seem to go with at least 400mg a day with doxy.
Post Edited (Spall) : 2/13/2012 12:28:47 AM (GMT-7)