Are there any known risks or side effects to taking green coffee bean supplements for weight loss?

Not studied. If you swallow anything that provides a stimulatory effect (caffeine products) or that induces mild nausea (coffee grounds), then you can expect to lose your appetite and therefore lose weight. These are not "fat burners" or "metabolism speeders" (there is no such thing), nor are they healthy. See your doctor for a healthy diet that is right for you, and stay away from snake oil.