“What is the fastest medicine for constipation?”

Every person is different, so what works fastest will not be the same for everyone. For general uncomplicated slow transit constipation, start with dietary changes (increased fiber, fruits, laxative teas, etc) then try over the counter laxatives like Miralax and dulcolax. If these do not help then Rx laxatives can be sought via a physician. The more aggressive one is with laxative use the faster they tend to work.