You said it correctly. The key word is "addicted." BATFE Form 4473 doesn't care why. If you require methadone you are addicted. If you check "yes" you'll be denied the sale. If you check "NO" you have offered a false instrument for filing with a government agency, a federal felony (see below).
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BATFE 4473, Question 11-e - Are you an unlawful user of, or addicted to, marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance? ___YES ___NO.
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I understand that a person who answers "yes" to any of the questions 11.b through 11.k is prohibited from purchasing or receiving a firearm. I also understand that making any false oral or written statement, or exhibiting any false or misrepresented identification with respect to this transaction, is a crime punishable as a felony under federal law, an may also violate state or local law.
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Specifically, 18 U.S.C. § 922(g)(1-9) prohibits the following from possessing, shipping/transporting, or receiving any firearm or ammunition: (3) a person who is an unlawful user of or who is addicted to a controlled substance;