Homegrown cannabis is estimated to be up to three times more potent than that imported from North Africa, the Caribbean and the Far East, it said.
The potency is measured by the presence of tetrahydrocannabinol, or THC, in cannabis, which decomposes over time making imported marijuana weaker than the homegrown variety.
On average, marijuana consumed in EU nations contains up to 8 percent THC, but in the Netherlands — which has a lax attitude toward soft drugs — the rate is double, said the study by the EU's Lisbon-based Monitoring Center for Drugs and Drug Addiction.
It said at least half of the marijuana smoked in the Netherlands is produced locally.
The Netherlands is already feeling the effects of the more-potent marijuana. Dutch officials say their policy of tolerance has not triggered higher drug use but raised concerns about health problems related to stronger cannabis.
Users of more powerful cannabis may be more prone to panic attacks and minor psychological problems, the EU study said.
It also said treatment for drug dependence is more becoming more "available, accessible and diverse" across the EU.
"Since we began monitoring in the mid-1990s, we have mapped a constant growth in all types of drug treatment in the EU," said Georges Estievenart, the drugs agency's chief.
Outpatient services rose in France by 25 percent, Greece by 30 percent and Austria by 60 percent in the past four years. In Britain and other countries waiting times for patients have fallen by as much as six weeks.
Also, addicts are now treated for different types of drugs, Estievenart said. "In the past, demand for drug treatment centered largely on opiate dependence. Nowadays, we are seeing more individuals seeking treatment for other substances, such as cannabis and cocaine."
In 2003, 410,000 people were used for heroin addiction, up from 320,000 in 1999, the report said.