Every other day is reasonable as long as you dont feel you want to go but cant pass anything. As long as the urge isnt there you are fine. Constipation is when you feel the urge but cant go and get cramps, pains etc...
If constipation is a regular thing and you end up buying stuff regularly go to the Drs, ask for what you have been buying to be put on prescription and something extra if it isnt enough. You are within your rights. They can do that, but if you are happy buying it they will let you. x