It's true enough that heavy drug use of one sort or another can interfere with employment earnings - but in any case benefits don't pay for much of a habit. Serious drug users get their money in other ways, frequently very socially destrictive ways. This is just one of the ways in which the "war on drugs" damages everybody.

Sorting out the drug problems of society isn't going to be significantly furthered by leaning on people living on benefits. Insofar as doing this increases pressures on people living on the edge and on their growing children, it could well make things worse.