The American Association of Poison Control Centers has reported zero deaths from vitamin C toxicity. Pharmaceuticals designed to reduce stomach acid, such as the proton-pump inhibitors (PPIs), are among the most widely sold drugs in the world. One PPI, omeprazole (Prilosec), has been found to lower the bioavailability of vitamin C by 12% after 28 days of treatment, independent of dietary intake. The probable mechanism of vitamin C reduction, intragastric pH elevated into alkalinity, would apply to all other PPI drugs, though not necessarily to doses of PPIs low enough to keep the stomach slightly acidic. In another study, 40 mg/day of omeprazole lowered the fasting gastric vitamin C levels from 3.8 to 0.7 µg/mL. Aspirin may also inhibit the absorption of vitamin C. There are regulations in most countries that limit the claims regarding treatment of disease that can be placed on food and dietary supplement product labels. For example, claims of therapeutic effect with respect to the treatment of any medical condition or disease are prohibited by the United States Food and Drug Administration even if the substance in question has gone through well conducted clinical trials with positive outcomes. Claims are limited to Structure:Function phrasing ("Helps maintain a healthy immune system") and the following notice is mandatory on food and dietary supplement product labels that make these types of health claims: "These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease." The use of vitamin C in high doses as a treatment for cancer was promoted by Linus Pauling, based on a 1976 study published with Ewan Cameron which reported intravenous vitamin C significantly increased lifespans of patients with advanced cancer. This trial was criticized by the National Cancer Institute for being designed poorly, and three subsequent trials conducted at the Mayo Clinic could not replicate the results. However, the trials at the Mayo Clinic were conducted with oral doses of ascorbate, not intravenous ascorbate as was done in the original trial, and it has been suggested that this difference is a possible reason for the failure of the Mayo Clinic trials to replicate the original trial results. Preliminary clinical trials in humans have shown that it is unlikely to be a "miracle pill" for cancer and more research is necessary before any definitive conclusions about efficacy can be reached. A 2010 review of 33 years of research on vitamin C to treat cancer stated "we have to conclude that we still do not know whether Vitamin C has any clinically significant antitumor activity.