I did a google search on st john's wort. I did not find any citations that recommended it for improving energy in diabetics. from what I've read, st john's wort is used to help people with depression, not low energy.
also, if a person is already taking prescribed medications, they must talk to their doctor before trying any sort of supplements. that's because supplements can and do interact with medications.
you might want to read these links on supplements www.mayoclinic.com/health/herbal-supplemen