Ghosties had a different blood biochemistry to humans. (AUDIO: Asylum)
According to the Seventh Doctor, the pH of the blood gave an idea of a species' basic biochemistry. (AUDIO: A Thousand Tiny Wings)
Elkinar Dajan was a doctor of biochemistry and applied xeno-agronomy. (AUDIO: The Seeds of War)
Hex noted that the Seventh Doctor had a different biochemistry to humans, and therefore using chloroform on him consistently was not having the "right" effect. (AUDIO: The Magic Mousetrap) Liz Shaw also said the same of the Third Doctor, but said that it didn't necessarily make him immune to the bacterial pathogen released by the Silurians in the 20th century. (TV: Doctor Who and the Silurians)