Oxytetracycline was the second of the broad-spectrum tetracycline group of antibiotics to be discovered. It is active against a wide variety of bacteria. Oxytetracycline works by interfering with the ability of bacteria to produce essential proteins. Without these proteins, the bacteria cannot grow, multiply and increase in numbers. Oxytetracycline therefore stops the spread of the infection and the remaining bacteria are killed by the immune system or eventually die. Side effects are mainly gastrointestinal and photosensitive allergic reactions common to the tetracycline antibiotics group. It can also damage calcium-rich organs, such as teeth and bones, although this is very rare. It sometimes causes nasal cavities to erode.