Working through the wizard to see if you're eligible to use the Visa Waiver Program (assuming that you have no criminal convictions), I've not encountered any references to drug addiction or rehabilitation schemes. (i.e. there's nothing to bar you from using the VWP):
http://www.usembassy.org.uk/cons_new/visa/visa _wizard.html
The US embassy website provides information about importing prescribed drugs, here:
http://london.usembassy.gov/ukcust1.html
I'm usually wary about directing people to unofficial (i.e. non-governmental) websites because of the possibility of inaccurate information. However, this site provides links to official US websites and tells you exactly what you need to print out in order to convince US customs that you can legally import prescribed methadone:
http://www.indro-online.de/usa.htm
For further information contact the London office of the US Department of Homeland Security, Customs and Border Protection on 020-7894-0771 (8.30a.m. to 4.30p.m.)
Chris