Adyson, it's an axiom that empirical research is more valued than people's comments in message boards, the kindly recommendation of friends and well-meaning, but poorly informed family members. It's like believing the magazines at the store checkout counter compared to Lancet, JAMA, Nature or National Geographic. Used as directed, you can literally use opioids for a lifetime and do no harm to your body.
Opioids will not harm your liver as long you have a healthy liver when you start and you use the meds as directed by your physician. In fact, it's the Acetaminophen or the aspirin that activates the opioid that's more harmful. Again, used as directed, you are safe.
The rate of addiction among chronic pain patients using opioids is below 2% (see
http://www.ncbi.nlm.nih.gov/pubmed/20091598?itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum&ordinalpos=1 and
http://updates.pain-topics.org/2011/01/study-finds-low-risk-of-rx-opioid-use.html . If you have no prior history of addiction disorder, you have nothing to fear.
Withdrawal is not a problem as long as it is under the guidance of a physician. The physician can administer an opiate antagonist to ease the way.
Tolerance is not a significant problem. When you reach the largest safe dosage of your medication, your physician will rotate you to a chemically different pain medication for two or three months. After that, you can return to the original medication with maximum efficacy.
Why do you think you need to switch to methadone? Is Fentanyl no longer helping? There are dosages ranging from 25 mcg to 100 mcg. Many of us here have been using powerful opioids, including Fentanyl, for more than 20 years with no problems at all. Are you prepared for the possibility that you will have a significant increase in pain? Fentanyl is about 80 times more potent than morphine. I hope this works well for you. But, realistically, there is a big difference in pain relief between the two drugs.
If your pain is lower now than it was before, perhaps you can get by with something less powerful than Fentanyl. But if your pain is the same, then switching to a less powerful medication could produce some unexpected results. I know that some of the things we hear or read can be frightening. But not all are to be believed. Best of luck to you.