The Home Office does not know whether a £1.2 billion-a-year strategy to tackle problem drug users reduced the costs of associated crime, a senior committee of MPs said.
The cross-party Public Accounts Committee (PAC) described the Government's failure to evaluate the overall impact of so much spending as "unacceptable".
The committee said there were 330,000 problem users of heroin and crack cocaine who cost society £15 billion a year, with crime accounting for £13.9 billion of that.
In a report, the PAC said: "The Government spends £1.2 billion a year on measures aimed at tackling problem drug use, yet does not know what overall effect this spending is having."
It added that the Home Office did not know whether a cross-Whitehall strategy launched in 2008 reduced the cost of crimes committed by problem drug users.
Nor could it "prove a causal link between the measures in the strategy and the levels of offending by problem drug users", the PAC said.
The Home Office has now agreed to publish annual reports from 2011 on progress against the 10-year strategy.
The PAC found that problem drug users typically relapse several times during or after treatment, and that about a quarter resisted any help at all.
Tory MP Edward Leigh, chairman of the PAC, said: "There are around a third of a million problem drug users in England who are costing society an estimated £15 billion a year - mainly as a result of their criminal activity.
"Central and local government spends around £1.2 billion a year on activities to tackle problem drug use. Given the amount of public money being spent, it is unacceptable that the Home Office does not know what overall effect this spending is having."