herbal laxitives dangerous?

I want to undertake a juice fast and I understand how important it is to cleanse the colon at a time when the colon shuts down because the body isn't taking in any solid foods, however, i was always told that when one takes a laxitive too often (especially strongs ones such as senna (e.g. smooth move)that one eventually becomes dependant on it and needs it to move the bowels all the time and that it's really unhealthy to use it unless absolutely necessary. The recommended dosage of senna is that it shouldn't be take more than once or twice a week so i don't understand how it's completely unharmful to be taking it at least once a day for thirty days straight....can someone please explain?