There are no major effects on metabolism from opiates; they tend to reduce heart rate in most people a small amount, and they may reduce cortisol release in response to stress during their initial use... but you become tolerant to all of the effects of opiates quite quickly and those effects--which are very subtle if at all-- go away after a couple days.
The constipation is something totally different-- opiates slow and even halt bowel motility through effects at the same mu receptor as is responsible for pain relief. Again, you become largely tolerant to those effects as well over time. A person can have a very fast metabolism and be constipated, and vice versa.
All of the effects of opiates are reversible-- save the damage to veins during chronic IV use, or the brain damage from a hypoxic arrest during overdose.
I average about 15 norco's per day, and I have been on vicodin for 17 years or more, what damage may I have already caused to my system, and can it be repaired?
How do I quit and remain pain free?
NO, they actually slow down your metabolism and make you constipated!!