Well all things are cleared from your blood by your liver so you would be making your liver have to work that much harder to clean an additional chemical from your blood stream.
Also some insurances won't prescribe hep c medications to those who use drugs. Your insurance may require abstinence from drugs for a period of time before they will approve treatment.
Is there a reason you feel you need to use marijuana?
Also if you drink alcohol you should stop drinking as hep c and alcohol work together causing more damage than they could alone.
It has been said that drinking while infected with hep c is like throwing gasoline on a fire.