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Lil Wayne said ICU later to Cedars-Sinai Medical Center in West Hollywood ... because TMZ has learned he was just released.

Wayne spent 6 days in the hospital -- most of it in the Intensive Care Unit -- after suffering what appears to be a drug OD that triggered a series of massive seizures.

Sources connected with the rapper tell us ... Wayne is feeling weak but he's a lot better.  He plans on laying low until he makes a full recovery.