Can i drink grapefruit juice one day before colonscopy?

This depends on what: other medications you are taking and on the colon preparation required by the gastroenterologist. Technically it would not be considered a "clear" liquid. It would be best to check with the health care provider performing the procedure as well as your primary health care provider regarding this concern.

Yes, a little: Clear liquids (water; fruit juices WITHOUT pulp -- apple, white grape; fruit flavored beverages -- fruit punch, lemonade; carbonated drinks; Jello; and tea/coffee without milk/cream) are recommended. So grapefruit juice is not technically a clear liquid. So have a glass if you must, but really better to go with the list above. Good wishes:)