The most common reason people get hooked on prescription drugs is because of the tendancy to take more than the prescribed dose.  The road to addiction ususally starts with a minor surgery and people abuse their prescription.  Once the prescription runs out, you have to get the drug somewhere.  Eventually people turn to hard core drugs like heroin to replace the feeling.  I know a few people that have started using heroin to wean themselves off of prescription drugs.  It starts on the weekends, then you move to two days in a row, and then everyday. 
    It's pretty impossible to end a heroin/prescription drug addiction on your own.  Withdrawals can get so bad that some people have even died because they've been living off of the opiate for so long.  A lot of people become suicidal not because they really want to die, but because they want a way out of the physical pain of withdrawals. 
    Methadone has actually saved a lot of lives and made it possible for people to function normally and make it through withdrawals.  Methadone is a mild form of heroin and you still can overdose on it.  You can be prescribed it on a weekly basis, but it's also addictive.  It's really just best to be responsible from the beginning when taking prescription drugs.  You can save yourself a lot of pain and money from simply avoiding addiction, withdrawals, and years of detox.

The problem with methadone is that most people aren't able to montior their doses by themselves.  Doctors don't take the time to monitor their patiets close enough.  You're giving a potentially deadly drug to addicts, assuming they're going to take the right amount.  There's been more methadone overdoses this past year than ever before. You make it seem like the perfect solution to prescription/heroin addictions, but it's not always the best solution.