Fiber that comes from whole foods is called dietary fiber. Fiber that's sold in supplements, or added to fortified foods, is called functional fiber. The Institute of Medicine has set an adequate intake (AI) for total fiber, which includes all sources. Getting this amount of fiber should be enough to stay healthy. Doctors may recommend higher doses of fiber.
Even in high amounts, fiber appears to be safe. Experts have not discovered an amount of fiber that's harmful.