WASHINGTON — Representative Trey Radel, Republican of Florida, who pleaded guilty in November to a misdemeanor charge of buying 3.5 grams of cocaine and was sentenced to a year of probation, resigned from Congress effective Monday evening.

Mr. Radel, 37, a freshman legislator, quietly returned to Congress in January after nearly a month at a private rehabilitation center in Naples, Fla., but he announced his resignation plans on Monday morning.

Referring to his “personal struggles,” Mr. Radel wrote to Speaker John A. Boehner of Ohio, “While I have dealt with those issues on a personal level, it is my belief that professionally I cannot fully and effectively serve as a United States representative to the place I love and call home, Southwest Florida.”

When he was arrested, in Washington, Mr. Radel said he struggled “with the disease of alcoholism.”

His plan to resign was first reported by Politico.

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