Family member having severe health problems after zithromax z-pak

trying to find out how to stop the downhill spiral of a mature family member. The only thing we can find relative to his immediate failed health was a prescription of Zithormax Z-Pak. Within a short time, he broke out in fever blisters, became anemic, dehydated, has the mental aptitude of a child, wets his pants and has severe short term memory loss. He has been taken from a mature intelligent adult to a mentally disabled child within a few days. He has no strength, is not hungry and cannot care for himself. Doctors have ruled out a stroke, cancer and organ failure. Trying to find some answer to help him. Thank you.