Hello, and I hope I can help you today. All birth control pills work to prevent pregnancy, so it is certainly safe and fine if you have decided to change brands. As long as you start your pack of Althea at the same time you would have started your Marvelon, then you will be protected against pregnancy. You may notice different side effects while taking the new pill while your body adjusts to the new formulation, especially in the first month of use. These symptoms may include nausea, headaches, spotting, bloating or breast tenderness. These will improve once you get your first period on the new brand. I hope I was able to adequately answer your question today and that my advice was helpful. Best wishes, Dr. Brown