Drug abuse is rife in the Afghan police force with up to four out of 10 recruits testing positive for illegal drugs in some areas, a US report says.
The report for the US Congress said the illegal drugs trade "undermines virtually every aspect" of efforts to secure Afghanistan.
Afghanistan produces 90% of the world's opium and the drugs trade is a key source of funding for the insurgency.
Another study found both cannabis and opium use was rampant among police across the country.
US police trainers found of 5,320 Afghan police and recruits given nationwide drugs tests, 16 per cent were found to be using drugs.
Gulab Mangal, governor of Helmand, admitted drug abuse was a "huge problem" among police in the province.
Coalition forces are battling militants in opium-producing areas of the south.
Southern drug belt
The reliability of the poorly paid Afghan police has frequently been questioned.
The Government Accountability Office (GAO) report highlights a number of serious shortcomings in Afghan police efforts in the fight against the illegal narcotics trade.
Some 12%-41% of police recruits in regional training centres tested positive for illicit drugs, say US state department officials cited in the report for Congress.
The report adds the figures are likely to underestimate the number of opium users because opiates leave the system quickly.
"Many recruits who tested negative for drugs have shown opium withdrawal symptoms later in their training," the report said.
It said the authorities are considering setting up rehabilitation clinics at regional police training centres - but there are questions over the long-term treatment of addicts once they finish their training.
In February 2009 an unnamed UK official estimated that 60% of Afghan police in the southern province of Helmand use drugs, emails obtained by the BBC showed.
Analysts say that the drug problem in the police is higher in the southern provinces where drugs are readily available.
The Government Accountability Office report also cited shortcomings in counter-narcotics police training which had resulted in "inconsistent" crime scene investigations and poor evidence gathering.
It said high illiteracy rates among counter-narcotics police added to the problem, as did "pervasive" drug-related corruption among local police and government officials.