Hello and welcome to health care magic community . Prednisone is a class of drugs called steroids . It has numerous side effects but these effects take quite sometime to appear . however , muscle weakness and lethargy is one of them . I think he's not been taking this medication for a long time and as soon as the medicine is stopped , lethargy may disappear , since mostly , it's reversible . Please no need to worry , but if lethargy continues even after cessation of treatment then you should find out the cause by seeing your primary , through a battery of tests , he / she may prescribe . Please also keep in mind that lethargy could be due to other reasons too , like lack of sleep , weather , and type of diet intake . Should use fresh fruit and vegetables too . A person eating and drinking well , means he may not have a serious issue . Hope I have answered your question . Any follow up questions will be welcome .