Why can't you take phentermine if you have glaucoma?

I have searched every where to find this answer. Every post says you can't take phentermine if you have glaucoma but it does not say why. I don't know that I have glaucoma for sure yet (one eye Dr thinks I do so I am being referred to another one) but I am going to see the eye Dr in a couple of weeks. I am currently on phentermine so I am curious what the medical reason is that you are not supposed to take it if you have glaucoma. Please help!