Research Triphala, it is a Three Herb Combination, used continuously for over 5,000 years. (extremely safe and has many side benefits which are outstanding)

Stay away from Senna, its very harmful and should only be used for emergencies.
Stay away from turkey rubarb same story

Cascara Sagrada is fairly safe, I wouldn't rely on it for a long time.

Many western herbs are misused, they are combined wrong. And without special harmonizing herbs, like Ginger, Fennel, Hing,  ect to ground out and prevent damage the primary herbs will damage the colon.

Taking Psyilium is safe, and some assisting herbs can be added.

Many of the stronger laxatives will disrupt the prana of the colon, and this can mean you are clean, and detoxed but in a less energized state than before. This will only cause recurrence of colon problems. So be careful what you take.

Im testing out a formula right now that puritanspride sells its fairly cheep, its mostly psyillium but has some assisting herbs which are combined in a safe way.

Good Luck!,

Eric
The Herbalist