> High concentrations of Al, Ca, Fe, Mg, Mn, and P were found, indicating that adulterants such as gypsum, marble powder, limestone, cement, and others must be used to increase the profitability of the illicit drug market.
http://static.sites.sbq.org.br/jbcs.sbq.org.br/pdf/2020-0204...
Diluting a very expensive product with something very cheap and selling it for the price of the pure product would certainly be a way of increasing profits.
From what I've read, many overdoses happen when users who are used to diluted heroin get hold of pure heroin, ending up with a much bigger dose than expected.
The paper I cited notes another risk for cocaine users: "Diluting substances such as talc, plaster, lime and chalk, used to increase the volume of the drug, are not absorbed and can cause chronic obstructive pulmonary diseases, such as silicosis".