honestly,i imagine you would have to go for an interview to the american embassy because they would want it declared that you were a recovering addict.best of luck

NO matter what the medicine, NEVER put it in your suitcase- always in your carry-on. If your luggage is lost...... well, you get the idea.
What types of medicine you use is of no concern to TSA.
What they are interested in is liquids.
If your meds are in liquid form and exceed the permitted amount you will have to present it at screening . Meds must be in an original container with the prescription number and the person's name on them. A letter from your Dr (in addition) is fine.
http://www.tsa.gov/travelers/airtravel/specialneeds/index.shtm

Thanks for the advice, its not actually for me just for someone I love dearly and will be travelling with me

Here's a couple of points

Emedia43 states no concern to TSA = Correct - however Customs will have a different view.

Metadone is (as you know) for drug adiction use, as a drug adict (or former drug user) you must get a visa as the Visa Waiver Programm excludes drug users. Call the US embassy in UK and ask if methadone is allowed accross the boarder (with or without a doctors letter) and if a Methadone user can enter the US under VWP.

tell the person travelling to speak to their prescriber, either GP or drug agency, they will know who to make contact with to sort you a licence to carry it out of the country. The problem may be with the USA and getting it into the country. it needs to be done in plenty of time. I used to be a drug worker and we used to sort it for clients.

hope it gets sorted quickly. hugs la x

This information may help.
http://www.deadiversion.usdoj.gov/21cfr/cfr/1301/1301_26.htm