WARSAW — In the year 2000, as the president of Poland, I signed one of Europe’s most conservative laws on drug possession. Any amount of illicit substances a person possessed meant they were eligible for up to three years in prison. Our hope was that this would help to liberate Poland, and especially its youths, from drugs that not only have a potential to ruin the lives of the people who abuse them but also have been propelling the spread of H.I.V. among people who inject them.

We assumed that giving the criminal justice system the power to arrest, prosecute and jail people caught with even minuscule amounts of drugs, including marijuana, would improve police effectiveness in bringing to justice persons responsible for supplying illicit drugs. We also expected that the prospect of being put behind bars would deter people from abusing illegal drugs, and thus dampen demand.

We were mistaken on both of our assumptions. Jail sentences for the possession of illicit drugs — in any amount and for any purpose — did not lead to the jailing of drug traffickers. Nor did it prove to be a deterrent to drug abuse.

What the law did do, however, was enable the police to increase their arrest numbers by hauling in droves of young people caught with small amounts of marijuana. More than a half of all arrests under the law were of people aged 24 and younger. Criminalization of drug users resulted in a dramatic increase in the number of identified cases of drug possession: from 2,815 in 2000 to 30,548 in 2008.