What clause in the Constitution allows the federal government to prohibit intoxicants?
None, but I’m not willing to subsidize someone who decides to sit around and be stoned or hung over all day. Hence my opinion.
The constitution DOES allow congress to enact laws. As such drug laws are statutory, not constitutional.
Elaboration via analogy,
There is nothing in the constitution, federal or state, that says I can’t take your property. The constitutions only prevents the government from taking your property without due process.
However, since the constitutions establish a congress that is granted the authority to enact laws, state congresses have made theft illegal.
The exact same clause that allows welfare programs of any sort. Including guaranteed medical care.
You get rid of those and drug addiction regulates itself, by terminating addicts through overdose (Or death when whoever they are robbing shoots them) at an early age. Relive the 1870s.