Commonly observed structural alerts that bioactivation can render toxic. Our furan, phenol, nitroaromatic, and thiophene structural alert evaluation sets contain 17, 188, 37, and 42 FDA approved or withdrawn small molecule drugs (in purple), respectively. Among those, 13 furan-, 127 phenol-, 31 nitroaromatic-, and 31 thiophene-containing drugs have been metabolically studied (blue and red). Of those metabolically studied drugs, the furan, phenol, nitroaromatic, and thiophene structural alerts are bioactivated, respectively, 23%, 26%, 51%, and 26% of the time. Nitroaromatics can include aromatic rings of any size, so only a fragment of the ring is visualized.