Concomitant administration with tizanidine is contraindicated. It is important that you use your medication exactly as directed - asthma or sulfite allergy. Skin and skin structure infections. Doxycycline may also be used for purposes not listed in this medication guide: bone and joint infections, itching of the rectum or vagina. Skipping doses may also increase your risk of further infection that is resistant to antibiotics. Doxycycline is a tetracycline antibiotic. It fights bacteria in the body. Make sure you understand what you need to store your medication properly. Shorter, higher dose treatments or uninterrupted double courses should be used only as a last resort. Take Viagra or Doxycycline on an empty stomach. Many antibacterials used in serious infections must be dosed intravenously. Tell your health care provider if any of these symptoms are severe or do not go away. It can be used in a treatment plan in combination with other agents, such as quinine. Its effects are systemic and nonselective. Side effects have been reported in two nursing infants.