My cat is constipated. Is I/D food effective, or what's a better diet?

There are many different diets to try for constipation. Hill's i/d is a highly digestible food that might result in less stool produced and help with constipation. Other foods like Royal Canin fiber response help increase the fiber in the stool, and sometimes canned high protein diets are recommended. Constipation generally occurs secondary to another disease process in cats like renal disease or megacolon, so the ultimate diagnosis might also influence what food was recommended. It would be a good idea to get full medical records and talk to your veterinarian at home about long term management once you are back in the country.