Tamiflu is an antiviral medicine and is mostly used to help you get better once you have already caught the flu. It shortens the duration of the symptoms and makes them less severe when taken within 48 hours of your first symptoms. So, the criteria for when you can consider yourself no longer contagious is the same with or without Tamiflu. But the length of time can vary from individual to individual. According to the CDC, you can consider yourself no longer contagious after 24 straight hours from your last fever (when not taking fever reducers). That will likely be sooner for someone taking tamiflu than it is for someone who is not.