New Delhi, July 18 -- Few products have captured public interest as fast as GLP-1 receptor-agonist weight-loss drugs. But one unforeseen consequence of their popularity has been to create what is, in effect, one of the biggest off-label medical experiments ever. This is the rapid spread of "microdosing": in other words, of taking less than the recommended amount.

Microdosing has become so popular (one estimate suggests one user in seven has indulged in it) that in America a whole industry has grown up around it. It is offered by big online firms such as Noom and Hims & Hers, is also available at lots of smaller medical spas dotted around the country and, most worryingly, is something people often just try for themselves at home.

Some mi...