Curcumin is the main bio-active constituent in turmeric. It has powerful anti-inflammatory properties, and there is evidence that it can help alleviate various conditions from chronic pain to depression. Because curcumin has poor bio-availability on its own it is often combined with black-pepper or with lipids.
Regarding possible adverse effects tests have shown that doses of up to 8 grams of curcuminoids have not been associated with serious adverse effects in humans.