GPs say they are coming under pressure from Primary Care Trusts (PCT) to reduce the amount of drugs supplied per prescription.
In some cases this means patients in England getting 28 days' supply instead of 56, doubling the prescription cost.
The Patients Association reports a rise in the number of calls to its helpline, and warns that some people may not be able to afford the rise.
The Department of Health said GPs were not obliged to follow PCT guidelines.
Many people who need long-term medication are used to getting prescriptions for their medicine that will last them two months. It keeps the costs down for patients in England who still have to pay and means they do not have to keep visiting their surgery.
Hazel from London relies on regular medication to reduce her cholesterol levels. She had a shock when she went to collect her latest prescription, and was told she could only have 28 pills instead of 56:
"I went to pick up a regular prescription. There was a note attached to it which said that this initiative which allowed me less pills was down to the local Primary Care Trust."
Hazel was upset that this change had come hard on the heels of prescription charges in England rising to £7.40 per item.
"I feel it's unjust. This means the prescription charge has gone up by 100%."
Hazel is not alone in being alarmed by this change in policy by some Primary Care Trusts. The Patients Association says it is receiving calls to its helpline from others complaining of similar rises.
Dr Mike Smith, an Association trustee, says forcing patients to buy more prescriptions for their condition could have serious consequences:
"I know from pharmacists that quite often the patient will say I'll do without that one and that one this month because I can't afford it. Now that is not right."
The Department of Health insists that although Primary Care Trusts can issue guidelines on the amount of drugs GPs prescribe, it is up to the individual GP whether he or she wants to follow them.
Hazel's Primary Care Trust, NHS Enfield, admits it changed its guidance to GPs in January but insists doctors were not obliged to follow it. When Hazel phoned the Trust on the advice of her GP, it said she would was able to get her 56 day prescription again after all.
The body that represents the trusts is the Primary Care Trust Network. David Stout, the PCT Network's director, says its members are not reducing the quantity of drugs prescribed as an indiscriminate way of cutting costs:
"The idea is to avoid waste. A parliamentary accounts committee estimated something like £100 million a year is wasted on medicines that never get used. "
But some of those writing the prescriptions wonder whether these new policies have been adequately thought through.
Dr Bill Beeby is the chairman of the clinical and prescribing committee at the British Medical Association and a GP in Middlesborough.
He believes his colleagues are coming under pressure to reduce prescriptions from 56 to 28 days, but in many cases no savings are being made at all:
"I can understand why practices are complying with PCTs because they're under considerable pressure to do so. But if patients are fully compliant with taking their medication and it is regularly taken and regularly needed then it saves absolutely no money."
The Department of Health says people requiring regular prescriptions in England who are not exempt can apply for a Prescriptions Prepayment Certificate. It costs £104 a year.