As Chris Bennetts mentioned Evan has Hep C and is a user of Interferon.
This prompted me to do some research and it does cause some Psychiatric disorders:
Any chronic illness, especially liver disease, may cause a person to become anxious and depressed. Furthermore, interferon itself may also cause or worsen underlying anxiety and depression. Interferon therapy has additionally been associated with causing irritability, confusion, emotional instability, insomnia and a lack or concentration.
Therefore, people who have a psychiatric disorder, or who are prone to depression, emotional instability or other related symptoms, should begin an antidepressant or antianxiety medication at least three weeks prior to commencing interferon therapy. In fact, it is probably beneficial for most people starting interferon therapy to begin an antidepressant or antianxiety medication.
All drugs are metabolized at least to some extent through the liver. While individuals with a chronic liver disease should avoid any non-essential medications so as not to subject their liver to additional stress, this directive may be outweighed by the great benefit to be obtained by treatment with an antidepressant or an antianxiety medication while on interferon therapy. Probably the safest antidepressants are the selective serotonin reuptake inhibitors (SSRI), such as Celexa (Forest Pharmaceuticals), Paxil (SmithKline Beecham), Zoloft (Pfizer) or Prozac (Dista/Eli Lilly). These typically decrease anxiety levels, but an additional antianxiety medication is sometimes needed such as BuSpar (Bristol-Myers Squibb). Individuals who develop any severe psychiatric side effects while on interferon are best managed jointly by a liver specialist and a psychiatrist. If severe depression associated with suicidal thoughts should occur, interferon therapy should immediately be discontinued.
Interferon may also cause thyroid abnormalities, symptoms of which may mimic psychiatric disorders. Individuals on interferon must have their thyroid profile periodically be checked via blood tests, especially when any of the above symptoms develop on therapy. Finally, all alcohol, nicotine and caffeine should be eliminated, as these substances may exacerbate anxiety and depression.