tract, the bacterial population of the intestine can become resistant to tetracyclines, resulting in overgrowth of resistant organisms. The widespread use of tetracyclines is thought to have contributed to an increase in the number of tetracycline-resistant organisms, in turn rendering certain infections more resilient to treatment. Tetracycline resistance is often due to the acquisition of new genes, which code for energy-dependent efflux of tetracyclines or for a protein that protects bacterial ribosomes from the action of tetracyclines. Furthermore, a limited number of bacteria acquire resistance to tetracyclines by mutations. Medical uses Tetracyclines are generally used in the treatment of infections of the urinary tract, respiratory tract, and the intestines and are also used in the treatment of chlamydia, especially in patients allergic to β-lactams and macrolides; however, their use for these indications is less popular than it once was due to widespread development of resistance in the causative organisms. Tetracyclines are widely used in the treatment of moderately severe acne and rosacea (tetracycline, oxytetracycline, doxycycline or minocycline). Anaerobic bacteria are not as susceptible to tetracyclines as are aerobic bacteria. Doxycycline is also used as a prophylactic treatment for infection by Bacillus anthracis (anthrax) and is effective against Yersinia pestis, the infectious agent of bubonic plague. It is also used for malaria treatment and prophylaxis, as well as treating elephantitis filariasis. Tetracyclines remain the treatment of choice for infections caused by chlamydia (trachoma, psittacosis, salpingitis, urethritis and L. venereum infection), Rickettsia (typhus, Rocky Mountain spotted fever), brucellosis and spirochetal infections (Lyme disease/borreliosis and syphilis). They are also used in veterinary medicine. They may have a role in reducing the duration and severity of cholera, although drug-resistance is mounting and their effect on overall mortality is questioned. Side effects Side-effects from tetracyclines are not common, but of particular note is phototoxicity. It increases the risk of sunburn under exposure to light from the sun or other sources. This may be of particular importance for those intending to take on vacations long-term doxycycline as a malaria prophylaxis. They may cause stomach or bowel upsets, and, on rare occasions, allergic reactions. Very rarely, severe headache and vision problems may be signs of dangerous secondary intracranial hypertension, also known as idiopathic intracranial hypertension. Tetracyclines are teratogens due to the likelihood of causing teeth discolouration in the fetus as they develop in infancy. For this same reason, tetracyclines are contraindicated for use in children under 8