Protocatechualdehyde (protocatechuic aldehyde) is a naturally occurring phenolic aldehyde that is found in barley, green cavendish bananas, grapevine leaves and root of the herb S. miltiorrhiza. There is evidence for anticancer activity of protocatechualdehyde in human colorectal cancer and breast cancer. Protocatechualdehyde leads to the inhibition of cell growth and the promotion of apoptosis through HDAC2-mediated cyclin D1 suppression in human colorectal cancer cells. In human breast cancer cells, protocatechualdehyde downregulates β-catenin and cyclin D1 through activating proteosomal degradation. The risk or severity of bleeding can be increased as a result of interaction of protocatechualdehyde with many drugs an dietary supplements (89 compounds reported).