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Stacker 2 diet pills are herbal dietary supplements promoted for weight loss. NVE Pharmaceuticals, which makes Stacker 2 and similar over-the-counter weight loss supplements, uses caffeine as this supplement's active ingredient. Caffeine probably won't give you the weight loss results you want, says the Center for Science in the Public Interest, or CSPI. And another ingredient in Stacker 2, bitter orange, may be unsafe to use.