Its online survey of 477 cancer patients found 44% were struggling to cope with drug costs.
The charity wants prescription charges abolished in England.
Prescription charges were scrapped in Wales in 2007 and will be phased out in Scotland by 2011.
Northern Ireland has frozen its charges while it considers whether to abolish prescription charges following a recent review.
In England, the government has ruled out any move towards free prescriptions.
Ministers are planning to launch a consultation in the near future, but this will only be looking at "cost-neutral" ways of tweaking the system, including who should be exempt from charges.
The Macmillan poll found 44% of patients had cut back on essential items like food or heating to pay for the cost of their prescriptions.
At the moment, people over 60, children and those on low incomes do not have to pay the £7.10 per item price tag.
In all, 88% of prescription items are dispensed in the community free of charge.
Macmillan says cancer patients can spend hundreds of pounds each year paying for prescriptions.
Nearly one in 10 cancer patients are unable to get all of the drugs prescribed to them due to the high costs of prescription charges, leaving some patients forced to select their medication based on what they can afford, rather than what their doctor prescribes.
On the fortieth anniversary of introducing the medical exemptions list for some patients, Macmillan Cancer Support is calling on the Government to reform the current prescription charging system to make it fair for all patients. For instance, at the moment people with diabetes get their prescriptions for free while most people under 60 with cancer have to pay. This is simply unfair to cancer patients who are also high users of prescriptions, the nonprofit said in April last.
Ciarán Devane, chief executive of Macmillan Cancer Support said,
"The current prescription charging system which gives medical exemptions to some illnesses but not others was devised forty years ago. It simply does not reflect the fact that cancer treatment has changed and most cancer patients now live for long periods with, and after, the disease. Patients tell me they are often spending up to several hundred pounds a year on prescriptions for drugs to cope with the side effects of cancer treatment.
"The reality today is that we have very ill people asking their doctor which drugs they can do without so they can bring down the cost of their prescriptions. Cancer patients should not be put in the position of having to forgo pain relief or other drugs to save money."
Cancer patients usually need multiple prescriptions to ease distressing side effects of cancer treatment like nausea, fatigue, severe mouth ulcers, and debilitating diarrhoea and can spend hundreds of pounds each year paying for prescriptions.
Barbara, 48, a mother from Norfolk was diagnosed with synovial sarcoma (soft tissue cancer in the ankle) in 1999. She says:
"Over the years I've had so many prescriptions that I've lost count. As a single mum struggling to work, support my son and pay the daily bills, there were many times when I couldn't afford to pay. Each time I picked up the prescriptions from the GP, I'd choose which drugs not to take because I couldn't afford the total expense. Who knows if that's the reason the cancer has kept coming back."
The Government has already said that the outcome of a review into prescription charges must be "cost neutral" - meaning it refuses to consider any option that involves spending even one extra pound on the total prescriptions budget.
Macmillan believes that no one should have to pay for their prescriptions - it is a tax on illness and we are disappointed with the Government's approach so far.
But a spokesperson for the Department of Health noted that prescription charges were a valuable source of income for the NHS in England.
Entitlement to free prescriptions in England is based on the principle that those who can afford to contribute should do so, while those who are likely to have difficulty in paying should be protected. There are extensive exemption arrangements already in place, it is argued.