In a double-blind cross-over study in seven patients, oral ferrous sulphate (80 mg Fe⁺⁺) given 3, 7 and 11 h after a daily oral dose of doxycycline, lowered the serum concentration of doxycycline by 20–45%. The half-life of intravenous doxycycline was shortened from 16.6±0.7 h to 11.0±0.4 h by concomitant oral iron therapy. These results are an indirect indication of significant intestinal secretion and reabsorption of doxycycline. The interaction between doxycycline and iron cannot be avoided completely by leaving an interval of 3 h between doses of the two drugs.