bacterial infection
"I have had Doxycycline 100mg for 7 days in two occasions, first for a gynecological infection and recently for sinusitis. In both cases, the drug had a very slow unset of action. The symptoms mostly disappeared on day 5 but the short treatment couldn't clear the infection. In the first occasion, I couldn't get it continued and got a bad recurring infection. This time I try a prolonged treatment.

I believe the reason is that doxycycline is a bacteriostatic agent and merely prevents bacteria from reproducing. The immune system itself should kill the bacteria. So, if your immune system is somehow weak it can't do the work in time. 

Anyway, doxycycline had a positive effect, even if it was prescribed empirically."