The Dark Side Of Curcumin, International Journal of Cancer, Online October 2009
Perhaps people say it is nontoxic because so little makes it into the bloodstream:
Several reports have demonstrated, however, that the plasma concentrations of curcumin in people taking relatively high oral doses of this compound are very low, typically in the nanomolar range.
For instance, a recent study examined the pharmacokinetics of a curcumin preparation in 12 healthy human volunteers 0.25–72 hr after an oral dose of 10 or 12 g. Using a high-performance liquid chromatography assay with a limit of detection of 50 ng mL1, only 1 subject had detectable free curcumin at any of the time points assayed.
One strategy for increasing curcumin’s bioavailability is to add piperine, an alkaloid found in black pepper (so, another isolated, concentrated substance), but:
This strategy, however, should be used cautiously, as piperine is a potent inhibitor of drug metabolism and may cause toxicity in people taking specific drugs. In addition, it is important to note that any strategy that increases the levels of curcumin in tissues will not only increase the effectiveness of curcumin, but also its toxicity.
DNA damage does not sound nontoxic:
A relatively high number of reports suggests that curcumin may cause toxicity under specific conditions. … Accumulating data have demonstrated since  that curcumin can induce DNA damage and chromosomal alterations both in vitro and in vivo at concentrations similar to those reported to exert beneficial effect. … These reports raise concern about curcumin safety, as the induction of DNA alterations is a common event in carcinogenesis.
Curcumin is sold as an antioxidant, but it can also promote oxidation:
Low concentrations of curcumin induce antioxidant effects, higher concentrations of this compound increase the cellular levels of ROS (reactive oxygen species).
Some other drawbacks:
Curcumin was recently found to be an active iron chelator in vivo and to induce a state of overt iron deficiency anemia in mice fed with diets poor in iron.
Curcumin has also been shown to inhibit the activity of the drug-metabolizing enzymes cytochrome P450, glutathione-S-transferase, and UDP-glucuronosyltransferase. The inhibition of these enzymes in people taking curcumin may lead to an undesired increase in the plasma concentrations of some drugs and cause toxicity.
People in India, according to these authors, consume only about 0.15 grams of turmeric a day, and it’s thought that amount contributes to their lower incidence of colon cancer. That’s 150 mg of turmeric, or about 5 mg of curcumin. Right? That is a whole lot less than the 2000 mg/day used in this pain study, and…
This dose of curcumin is similar to that recommended by the World Heath Organization, but ~10 times lower than that generally recommended by suppliers of curcumin supplements.
It is unfortunate that curcumin is regarded in the scientific literature as efficient and safe when its efficiency and safety have not yet been proven.
Curcumin is not nontoxic.