Heroin use is stable or declining in most of western Europe, but increasing in eastern Europe and the former Soviet Union, says the UN's drugs watchdog.
In its annual report, the International Narcotics Control Board said Russia was the biggest heroin market in Europe.
It added that the UK had the largest heroin seizure rate in Europe and the third highest number of heroin addicts.
The board raised concerns that the recent EU enlargement could make life easier for drug traffickers.
EU membership increased from 15 to 25 last May.
"The board is concerned that the enlargement of the EU membership may lead to a weakening of existing import or export controls throughout Europe," the agency said.
"Several member states in the EU are... in the process of reorganising their customs services and significantly reducing their staff."
Policy change
The INCB added that such "adjustments should not have any adverse impact on the control of illicit drugs... and the capacity to act against drug trafficking".
The agency welcomed a decision by the Netherlands to modify its drug policy especially by reducing the number of coffee shops selling cannabis.
Other issues outlined in the report included:
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