Very bad. People are injecting them as well and over time the fillers they use in the pills can cause vein blockages as well as in the lungs and both can be fatal.
Reduce pain. As a result, the human body grows more dependent on pain pills when it encounters pain. Pain resistence decreases. When pain is experienced again, the human body is more sensitive to it...
As someone who does this on the regular (ill snort about 15 percosets at a time) and doing plenty of research i can tell u that the desired effect is that it will break down the active ingredient...