A law that became active on July 1, 2001 did not legalise drug use, but forced users caught with banned substances to appear in front of special addiction panels rather than in a criminal court.
The panels composed of psychologists, judges and social workers recommended action based on the specifics of each case.
Since then, government panels have recommended a response based largely on whether the individual is an occasional drug user or an addict.
Of the nearly 40,000 people currently being treated, "the vast majority of problematic users are today supported by a system that does not treat them as delinquents but as sick people," Goulao said.
In a report published last week, the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) said Portugal had dealt with this issue "in a pragmatic and innovative way."
Drug use statistics in Portugal are generally "below the European average and much lower than its only European neighbour, Spain," the report also said.
"The changes that were made in Portugal provide an interesting before-and-after study on the possible effects of decriminalisation," EMCDDA said.