Is it safe to take citrucel daily and long term? if so, why does the manufacturer say, laxative products, including citrucel with methylcellulose fiber, shouldn't be used for longer than one week unless you're directed to do so by your physician?

Yes, it is safe in general. The instruction is just to make sure you are evaluated by a doctor for ruling out some uncommon causes of constipation such as colon lesion such as colonic tumor, neurologic condition or even severe fecal impaction, etc. Remember, high fiber intake has to go along with high fluid intake when treating constipation. Take care.