Maximum effective dose of doxycycline is 40mg a day
There has been a lot of copy written about so-called low dose or anti-inflammatory dose doxycycline. This form of antibiotic therapy is one of the more promising recent therapeutic developments; it will likely become a widely known and available treatment.

This recently published paper shows that a daily dose of 40mg of doxycycline is as effective in reducing the skin lesions of rosacea as a higher dose of 100mg or 200mg a day.
Note that this is a statistical study using existing clinical trial data, and that the authors work for Collagenex who make Oracea.

Anti-Inflammatory Dose Doxycycline (40mg Controlled-Release) Confers Maximum Anti-Inflammatory Efficacy in Rosacea.
Skinmed. 2007

Background. Two large clinical trials have recently demonstrated the efficacy of a 40-mg controlled-release formulation of doxycycline in the treatment of rosacea, a dose well below the conventional level of 100 to 200 mg/d. Since no formal dose-response studies have been conducted, the authors analyzed phase 3 data to determine whether a dose-efficacy relationship exists.

Methods. Standard parametric regression analyses were used to estimate the correlations between dose (mg/kg body weight) and overall drug exposure (area under the curve [AUC]) in a phase 1 pharmacokinetic study and between dose and efficacy (mean change from baseline in total inflammatory lesion count at week 16) in 2 pooled phase 3 clinical efficacy studies. Additional regressions were run at each visit for the clinical efficacy studies to determine whether results differed across visits. A regression analysis was also performed in a subset of patients who showed a greater efficacy response.

Results. We found overall drug exposure (AUC) to have a highly significant correlation with dose (mg/kg) (r=0.49; P=.006). In contrast, clinical efficacy did not correlate with dose at any of the visits at week 3 (r=0.01; P=.85), week 6 (r=0.04; P=.53), week 12 (r<0.01; P=.98), and week 16 (r=0.03; P=.64) or among the subset of patients who showed greater clinical benefit.

Conclusions. Higher mg/kg doses led to higher plasma concentrations but did not lead to increased clinical efficacy. Anti-inflammatory dose doxycycline (40-mg controlled-release formulation) conferred peak anti-inflammatory efficacy in the treatment of rosacea.