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."