what are the symptoms or side effects to interferon treatment?

Fatigue -
patients say this is the most disruptive side effect of all.  Tiredness often carries on after treatment has ended.  Most people find their energy levels are back to normal from 6 months to a year after their treatment finishes.  

Flu-like symptoms:
 fever, chill, headache, aching muscles and joints.  Nearly everyone taking interferon has this side effect at first.  It usually gets better as your course of treatment continues.  These symptoms usually begin 2-4 hours after your injection and last for about 12 hours.  Take paracetamol before your injection and every 4-6 hours afterwards for as long as you need to (take no more than 8 tablets in 24 hours).  You can try having your injection and paracetamol before you go to bed to see if that helps.  

Feeling or being sick
this happens to about half the people treated

Diarrhea
you should drink plenty of fluids.  If it becomes severe or persistent you could become dehydrated so you should tell your doctor or nurse.

Loss of appetite
may increase as your course of treatment goes on.  You may also have taste changes or a metallic taste in your mouth  

Temporary effect on the bone marrow.  
The bone marrow makes blood cells and a drop in its function can cause