Doxycycline is a semi-synthetic, broad spectrum tetracycline antibiotic derived from methacycline. Tetracyclines are derivative of polycyclic naphthacenecarboxamide. They are bacteriostatic and effective against many gram positive and gram negative bacteria. Doxycycline is the first drug of choice for prophylaxis of "travelers diarrhoea".Doxycycline is administered orally. Antibiotics require constant drug level in body for therapeutic effect. This is achieved by taking the drug at regular interval of time throughout the day and night as prescribed. Doxycycline (HCl) is important to take the drug for the full time period as prescribed. If you discontinue drug, it may result in ineffective treatment.