Rooney Mara and Channing Tatum in “Side Effects.”

“Side Effects,” opening Friday in Boston, is a psychological thriller set amid psychiatric offices and high-stakes pharmacology. There is a plot twist, then another, then, oops, Soderbergh has said too much already. The Oscar winner for 2000’s “Traffic” doesn’t want to give the quote that gives away a single one of the surprises in what he insists will be the last movie he makes, at least for some time.