Each sitting member of congress should be drug tested annually at a random time during the year.
It is simply a national security issue. Sitting congress members have regular access to classified and other material, non-public, information. Drug use vastly increases the likelihood of the accidental or coerced release of such information to either foreign actors or those who may otherwise profit from the information. Just as every member of the military is annually tested, congress must also be tested for the security of our nation.
The cost will be minor compared to the costs of testing welfare recipients and the military already tests each member annually providing an economy of scale and protocol.
It is already legal to test sitting members of congress, it just needs to be done.