There's some good analysis on differences between the Portuguese and Oregonian approaches. This
NYT article for example noted some of the unique differences: for example, Portugal, while ending prison sentences for drug possession, still used a variety of other threats to "encourage" users to quit. Oregon effectively removed any consequences whatsoever. A quote given the firewall:
Drug addiction is an illness, but it is different from many other illnesses in a crucial respect. Most people with diabetes or cancer wish they could make their diseases disappear. Addicts have a more complex relationship with their disease. People with addiction often do not want treatment. They frequently think they have a handle on their drug use. That attitude is at the root of many people’s addictions.
“You need to answer the question: Why would people stop using an incredibly rewarding drug if there is no real consequence at all?” said Keith Humphreys, a drug policy expert at Stanford University.
A crucial part of Portugal’s change in 2000 was its attempt to nudge people to stop using drugs. The country did not simply decriminalize the substances. It also set up new incentives for seeking help: People caught using drugs can be sent to a special commission that tries to get them into free treatment. If drug users do not cooperate or they show serious problems, the commission can impose penalties, such as barring people from taking some jobs or visiting certain locations. It is a carrot-and-stick approach.
Oregon does not have much of a stick... As a result, people continue to use drugs, without an incentive to seek help. The implication here — that law enforcement matters for stopping addiction — might make some liberals uncomfortable. But the evidence strongly suggests that people with addiction often need a push to seek help. By ending the threat of arrest or prison time and not creating anything like Portugal’s commissions, Oregon was left without a push.
The article goes on to say that as Portugal started to cut funding to commissions and treatment, drug use started to grow again.