Is it bad to have left my tampon in for 12 hours and this is my first time leaving it in long

Change tampons often: It is important to change tampons every 4 to 8 hours. Leaving a tampon in longer is associated with a rare condition called toxic shock syndrome. Toxic shock syndrome symptoms include sudden fever, vomiting and diarrhea, sunburn like rash on hands and feet, red eyes, mouth and throat. Seek out immediate medical attention if any of these symptoms occur. More about toxic shock syndrome: http://www.webmd.com/women/guide/understanding-toxic-shock-syndrome-basics#1

Not good: This time is probably not a problem to have left the tampon in for 12 hours, but Toxic Shock Syndrome from prolonged indwelling tampon use with bacterial growth is no myth. It is a serious life threatening problem when tampons are "forgotten" or left in for extended periods of time. Sanitary products are relatively cheap and changing them frequently is your best defense, saturated or not.