People who cannot work because they are obese or have alcohol or drug problems could have their sickness benefits cut if they refuse treatment, the PM says.
David Cameron has launched a review of the current system, which he says fails to encourage people with long-term, treatable issues to get medical help.
Some 100,000 people with such conditions claim Employment and Support Allowance (ESA), the government says.
Labour said the policy would do nothing to help people to get off benefits.
Campaigners said it was "naive" to think overweight people did not want to change their lives.
There is currently no requirement for people with alcohol, drug or weight-related health problems to undertake treatment.
'A life of work'
Mr Cameron has asked Prof Dame Carol Black, an adviser to the Department of Health, to look at whether it would be appropriate to withhold benefits from those who are unwilling to accept help.
Announcing the proposal, he said: "Some [people] have drug or alcohol problems, but refuse treatment.
"In other cases people have problems with their weight that could be addressed - but instead a life on benefits rather than work becomes the choice.
"It is not fair to ask hardworking taxpayers to fund the benefits of people who refuse to accept the support and treatment that could help them get back to a life of work."
etcBy political correspondent Alex Forsyth
David Cameron sees the wide-ranging welfare reforms introduced in this Parliament as part of a "moral mission".
He has said they give new hope to people who have been written off by helping them back to work.
He also knows taxpayers who fund the welfare state like policies which ensure benefits only go to those who need them.
So despite criticism of what some see as an increasingly punitive benefits regime, the Conservatives are floating a new suggestion - possible sanctions for those claimants who refuse help to overcome treatable conditions.
On the same day, during a speech in Wales, Labour's leader will pledge to continue his attack on tax avoidance.
So David Cameron runs the risk of being seen as someone wanting to crack down on some of society's most vulnerable, while Ed Miliband targets the wealthiest.
The truth is both party leaders are trying to persuade "hard-working families" that they're on their side.
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Similar proposals have been considered by the government before.
In 2010 and 2012 the Conservatives considered plans to remove or cut benefits for drug and alcohol addicts who refused treatment.
At the time the plans were met with concern by charities, who said there was no evidence benefit sanctions would help addicts engage with treatment.
Note it also mentions drugs, I know a lot of you are weed smokers so will probably develop mental illnesses at some point in the future too. I think this is a good idea.
