There are several types of interferons. I guess you were prescribed interferon alfa-2a for Hepatitis C. One of its side effects is loss of scalp hair (alopecia). It is quite possible that there would have been fall of hair even without medication but it got worse.
While using potent medicines one has to keep in mind the benefit-risk ratio. In your case the use of interferon is essential more so when you are on phenytoin that has an adverse effect on the liver.