I do this. I have a favourite bottle and I carry it around the house with me & usually fill it about 3/4 times a day (as well as the odd tea & coffee). I don't want to change it as I don't like new things (my bottle's well 'used' in appearance). My friend saw it today & said I need to change it for a glass bottle asap because it really isn't healthy. So if I keep this long term am I really doing myself harm?