Propyl glycol is a colorless and odorless liquid that may be derived from petroleum, natural gas, or vegetable sources. The propyl glycol in Curcumin Plus is plant-based from vegetable sources. As with all our ingredients, we source plant-based ingredients when available as an option. Propyl glycol has a long history of safe use in personal care products. It is on the FDA list of ingredients that are Generally Recognized as Safe (GRAS). Propyl glycol in Curcumin Plus is an excipient or inert ingredient and thus is used on the machinery that the tablets slide down onto the bottle. It is not a major or minor ingredient but an inert/excipient ingredient that is so small it cannot be measured. However, unlike most companies, we disclose everything the product comes in contact with since it is used in hospitals, universities, and clinical trials.