Cancer patients often spend £100 a year on prescriptions
Nearly two thirds of the 150,000 cancer patients in England have not applied for free prescriptions - five months after they became available.
The £7.20 prescription charge was abolished for cancer patients after decisions in the rest of the UK to scrap all fees.
People have to fill in exemption forms to qualify, but charities said GPs were not promoting the scheme.
Doctors' groups responded by saying bureaucracy was putting patients off.
The five-year exemption forms are only one page long and would save the average cancer patient £100 a year.
But the British Medical Association believes many people are too consumed by battling cancer to deal with the bureaucracy.
Dr Richard Vautrey, of the BMA, said: "This is why we have always said England should have followed the lead of the rest of the UK and just lifted the charges altogether.
"Patients just cannot face filling out papers when they have cancer."
The latest Department of Health figures show that nearly 60,000 patients have applied for free prescriptions.
But Macmillan Cancer Support said it was concerned that not all GPs were aware of the scheme and that it also covered treatments for conditions such as depression that were linked to a patient's cancer.
Mike Hobday, head of campaigns and policy at the charity, said: "More than four months after the introduction of free prescriptions, it's worrying that the overwhelming majority of cancer patients are still scrimping and saving to pay for their medication.
"We urge GPs and pharmacists to check people collecting medication for cancer are aware of their entitlement."
Helen Parker, a 53-year-old from Kent, who has to take three drugs every day to ease the side-effects of her bowel and womb cancer treatment, agrees doctors are the problem.
"It was a real struggle to get an exemption form as the surgery didn't know about them and then I had to wrangle with my GP to get it accepted.
"He'd decided I wasn't eligible because I didn't have cancer at the time and that the drugs weren't for the cancer. Someone less vocal and more vulnerable than myself would have backed down and still be paying, or not taking much-needed drugs due to the high cost."
Free prescriptions for cancer patients were announced last September by Gordon Brown at the Labour Party conference.
It came after controversy about the position in England following changes elsewhere in the UK.
Charges have already been scrapped in Wales for all patients and are being phased out in Scotland and Northern Ireland.
Phil Hope, Health Minister for England, said: "Free prescriptions for cancer patients are right and fair.
"That's why we have taken action so that 60,000 cancer sufferers no longer have to worry about paying for their drugs.
"We are concerned that some cancer patients haven't yet taken advantage of this new right and that is why we need to continue to raise awareness amongst patients and to make sure that all GPs are offering this new entitlement to their cancer patients.
"GPs have a pivotal responsibility and I would urge them to make sure their patients are fully informed of their new rights to free prescriptions."