The liver is the gateway from the digestive system to bloodstream AND it is a filter for the bloodstream. If you take winny orally, the first pass is from the digestive system through the liver, then about every 30 minutes or so whatever remains passes through the liver with the blood until it is all deactivated.
If you inject, it still passes through the liver with the blood about every 30 minutes until it is all deactivated. Either way your liver has to deactivate all of it.