To defend their profits, the drug companies have warned Canadian wholesalers and pharmacies not to sell to Americans by mail, and are cutting back supplies to those who dare.
主干：...the drug companies have warned wholesalers and pharmacies not to...and are cutting back supplies...
句首的不定式to defend...作目的状语。句子含and链接两个并列的谓语动词构成复合谓语。前一个谓语是warn...not to...，不定式to sell作宾语补足语。
译文：为了保护他们的利润，医药公司已经警告加拿大的批发商和药房不要通过邮寄卖给美国人药品，并切断那些敢卖药给美国人的批发商和药房的供货。