Drugs kill brain cells, and brain cells never come back.

The second half of the claim was once standard neuroscience. Charles Gross, reviewing the history in 2000, wrote that for over a century the field assumed that "new neurons are not added to the adult mammalian brain".[1]

The first half was told above all about alcohol. In 1993 two Danish researchers opened their paper by noting that "it is commonly believed that chronic alcohol abuse results in loss of neocortical neurons", and that the assumption "has not been properly tested".[2]

They tested it. Jensen and Pakkenberg counted neurons in the neocortex of 11 alcoholic men and 11 matched controls after death. The "total mean neocortical neuron numbers in the two groups did not differ".[2] What the drinkers had lost was white matter: "chronic alcoholics lose white matter", the authors wrote, and they argued this damage may be reversible with abstinence.[2]

The same paper is also why the result matters. Its authors stated that "lost neocortical neurons cannot be replaced".[2] For most of the cortex, that half of the legend holds.

The hippocampus is different. In 1998 Eriksson and colleagues found newly divided neurons in the brains of adult patients and concluded that "the human hippocampus retains its ability to generate neurons throughout life".[3] A 2013 study dated neurons by the carbon-14 left from nuclear bomb tests and estimated that "700 new neurons are added in each hippocampus per day".[4]

The point is disputed. In 2018 Sorrells and colleagues found no young neurons in adult hippocampi and concluded that neurogenesis there "does not continue or is extremely rare in adult humans".[5] The same year Boldrini and colleagues examined hippocampi from people aged 14 to 79 and reported "thousands of immature neurons in the DG" at every age.[6]

The slogan fails on the case it was built on, since the best-known count in heavy drinkers found no lost cortical neurons.[2] Its second half is right for the cortex and too absolute for the hippocampus.[2][4]