The patch is more than 99 percent effective in preventing pregnancy when worn appropriately. Each patch is good for one week. Women should not wear the patch for longer than that or they will begin to show signs of removal. Transdermal patches are the most effective method of birth control available. When used as directed, they can protect against pregnancy for up to a year. Transdermal patches work by delivering hormones into the blood stream through the skin. The patches are applied once a week at the same time each week. Some women may experience some skin irritation when using this type of patch, especially if they have sensitive skin. If you do experience any skin problems while using the patch, apply a new one to continue protecting yourself from pregnancy.