When drugs are administered orally, absorption is much more complex than when administered intravenously. The drug has to bypass a number of barriers before it can get into the blood stream and do its thing. First, it must get past the acid in the stomach. Second, it must over come bacteria in the small intestines. Third, it must survive changes in the small intestines. Finally, it must survive the metabolism process in the liver known as the first pass effect.
During the first pass effect, blood travels to the liver as part of a filtering system where enzymes in the liver metabolize or change part of a drug before it enters the bloodstream. Enzymes are proteins that change drugs in the body and also help biochemical reactions occur as discussed above.
Drugs are absorbed faster when administered intravenously. They enter the blood stream immediately, and aren’t absorbed like when taken orally. They bypass being metabolized by the liver, skipping the first pass effect.
I had trouble finding this excerpt but a much better awnser than my first
also do a google for the specific compound you are refering to as many orals have different levels of toxicity