Doses of methadone are prepared for distribution to patients at the Taipas rehabilitation clinic in Lisbon, Portugual. Lisbon's Taipas rehabilitation clinic is one of over 40 such clinics across the country, the result of Portugal's 15-year-old pioneering drugs policy which decriminalised drug use while offering free treatment and state-funded street-work programmes.

OPINION: Portugal decriminalised the use of all drugs in 2001. Weed, cocaine, heroin, you name it -- Portugal decided to treat possession and use of small quantities of these drugs as a public health issue, not a criminal one. The drugs were still illegal, of course. But now getting caught with them meant a small fine and maybe a referral to a treatment program -- not jail time and a criminal record.

Whenever people debate drug policy - marijuana decriminalisation, for instance - many drug-policy makers predict dire consequences. "If you make any attractive commodity available at lower cost, you will have more users," former US Office of National Drug Control Policy deputy director Thomas McLellan once said of Portugal's policies. Joseph Califano, founder of the Center for Addiction and Substance Abuse at Columbia University, once warned that decriminalisation would "increase illegal drug availability and use among our children".

But in Portugal, the numbers paint a different story. The prevalence of past-year and past-month drug use among young adults has fallen since 2001, according to statistics compiled by the Transform Drug Policy Foundation, which advocates on behalf of ending the war on drugs. Overall adult use is down slightly too. And new HIV cases among drug users are way down.

RAFAEL MARCHANTE / REUTERS A patient drinks his doses of methadone at a rehabilitation clinic in Lisbon in this 2012 file photo.

Now, numbers just released from the European Monitoring Centre for Drugs and Drug Addiction paint an even more vivid picture of life under decriminalisation: drug overdose deaths in Portugal are the second-lowest in the European Union.

Among Portuguese adults, there are three drug overdose deaths for every 1,000,000 citizens. Comparable numbers in other countries range from 10.2 per million in the Netherlands to 44.6 per million in the UK, all the way up to 126.8 per million in Estonia. The EU average is 17.3 per million.

Perhaps more significantly, the report notes that the use of "legal highs" - like so-called "synthetic" marijuana, "bath salts" and the like - is lower in Portugal than in any of the other countries for which reliable data exists. This makes a lot of intuitive sense : why bother with fake weed or dangerous designer drugs when you can get the real stuff? This is arguably a positive development for public health in the sense that many of the designer drugs that people develop to skirt existing drug laws have terrible and often deadly side effects.

RAFAEL MARCHANTE / REUTERS Patients wait to take a blood test at the Taipas rehabilitation clinic in Lisbon, where addicts are treated instead of jailed.

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Drug use and drug deaths are complicated phenomena. They have many underlying causes. Portugal's low death rate can't be attributable solely to decriminalisation. As Joao Goulao, the architect of the country's decriminalisation policy, has said, "it's very difficult to identify a causal link between decriminalisation by itself and the positive tendencies we have seen".

Still, it's very clear that decriminalisation hasn't had the severe consequences that its opponents predicted. As the Transform Drug Policy Institute says in its analysis of Portugal's drug laws, "The reality is that Portugal's drug situation has improved significantly in several key areas. Most notably, HIV infections and drug-related deaths have decreased, while the dramatic rise in use feared by some has failed to materialise."

As legislatures debate with issues like marijuana legalisation and decriminalisation in the coming years, Portugal's 15-year experience may be informative.

Christopher Ingraham writes about politics, drug policy and all things data. He previously worked at the Brookings Institution and the Pew Research Center.