There is a question in the app that asks you if you applying to any other program. So, yes, you must tell them. In my opinion, it looks like you are not very serious about dentistry, but I'm sure others have done it.
Have you applied to pharmacy school, or are you applying to pharmacy school? Big difference
If you can articulate why you want to be a dentist well and have a solid reason for changing your mind to dentistry, then I wouldn’t think it would affect you that much