Drug test them and reduce welfare benefits if they get fired for drugs. Give them a few chances.
Tough love.
One reason why people are unwilling to be “tough” in that manner is because the fate of others (family) who did not abuse and were not in a position to be held guilty of laziness will be involved.
Generally, participation in a substance abuse recovery plan is required in states that have gone down this road. Some states have dropped it later due to finding the program financial costs are higher.