The conference in Vienna was to review the results of a decade-long war on drugs launched by a UN General Assembly session (Ungass).
'Problem not reduced'
A European Commission report published on Tuesday said it had "found no evidence that the global drug problem was reduced during the Ungass period from 1998 to 2007".
"Broadly speaking the situation has improved a little in some of the richer countries while for others it worsened, and for some it worsened sharply and substantially, among them a few large developing or transitional countries," the EC report said.
"In other words, the world drugs situation seems to be more or less in the same state as in 1998," the report by the 27-nation European Union's executive body said.
And while increased action had been taken against producers and sellers, the report noted that prices of narcotics in Western countries "have fallen since 1998 by as much as 10 to 30 per cent" and that there was "no evidence that drugs have become more difficult to obtain".
The EC report said enforcing drug bans had backfired by displacing drugs traffickers to relatively lawless regions.
The bans had led to addicts sharing needles – spreading disease such as Aids - as syringe-exchange centres had been unavailable.
Costa also underlined the need for harm-reduction measures, such as needle and syringe exchange programmes, substitution treatment of drug addiction and outreach work.
Backing up that position, Phillip Owen, a former mayor of Canadian city Vancouver, told Al Jazeera that drug policies based on law enforcement had "failed, everywhere, totally failed".
Owen, who was one of the first Canadian politicians to advocate a rethink of traditional anti-drug policies, said "the ideology saying that this is a criminal issue has absolutely not worked anywhere for decades and decades".
Saying that policy-makers must "accept that the user is sick and the dealer is evil", he called on them to look at scientific and medical evidence.
"Dozens and dozens of reports written all around the world from a scientific and medical point of view ... support drug policy reform, not prohibition," he said.