The warning comes as prescription charges in Scotland were poised to fall to £3 in April, after regulations on the issue were laid out last week.
Last year, prime minister Gordon Brown revealed plans for prescription charge exemptions for long-term condition patients in England.
Although the DoH asked the Royal College of Physicians' president Professor Ian Gilmore to lead a review into how exemptions could work, that review has yet to be published.
Asthma UK is leading the Prescription Charges Coalition, a group campaigning for the government to abolish prescription charges for people with long-term conditions.
The charity's director of policy and public affairs, Mikis Euripides, told GP that the government needs to publish Professor Gilmore's review and details of its plans early this year if the changes are to take place.
'The government will need to introduce legislation,' he said. 'Assuming there is a May general election, it will need to do that by February because of the time it will take. We are concerned that if nothing happens, the government won't have enough time.' If the election were earlier than May, it would already be too late to put legislation through, he added.
If the government does not draft legislation in time, there may still be a commitment in the Labour Party manifesto. However, any commitment would need to stick to Mr Brown's original commitment to be fair, Mr Euripides said.
He added that he would be 'shocked' if the Conservatives made any such commitment.