The charity, which conducted a survey of 2,004 adults, found that 39 per cent of cancer patients said they had looked online for information about their diagnosis, of which more than a quarter (27 per cent) reported it having a detrimental impact.
This figure increased to two in five (39 per cent) of those who turned to the internet within moments of their diagnosis.
Overall, one in 10 of the cancer patients involved in the study (11 per cent) said they had been negatively affected by looking up their diagnosis online.
When the results are scaled up to reflect the UK cancer population, it suggests more than 300,000 people could have experienced similar feelings.
As a result of the findings, Macmillan has issued a stark warning against looking up cancer diagnoses online, which it states could be due to understaffing and unmanageable workloads hindering frontline staff.
Dr Rosie Loftus, chief medical officer at Macmillan Cancer Support said: “In today’s digital society, it is understandable that people look up their diagnosis online when they’re told they have cancer.
“However, it is extremely concerning that such a high number of people who turn to the internet for support are presented with such a seemingly negative outlook.
“It is vital people with cancer are supported from day one. This not only depends on the cancer workforce having the time and capacity to fully explain what a diagnosis means, but also signposting people to reputable sources to ensure they start their cancer experience on the right foot.”
Among those surveyed, the top reasons for looking up their condition online were to look for more information and to find out about their prognosis, Macmillan said.
Five per cent of respondents said they had found bogus cancer cures online, with the same number adding that they thought they were going to die after searching the web.
When put in context of the UK cancer population, this could be more than 50,000 people for each scenario, Macmillan said.
Professor Peter Johnson, NHS clinical director for cancer, said: “Going online is a quick and popular way to find information, but it is vital that internet and social media platforms help people find reliable sources of information, and not those promoting myths and ‘miracle cures’ which at best don’t help, and at worst put people’s lives at risk.
A cancer diagnosis can be a huge shock. There are often so many emotions to deal with, and it can be difficult to know what you need to do next.
But we're here to help make sense of things. Get tailored information and support from day one 👉
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“As part of our long-term plan we are rolling out a faster diagnosis standard so that people get certainty sooner, and investing in more clinical nurse specialists and other professionals to ensure people get more personalised cancer care, including information they can trust.”
Macmillan Cancer Support provides an Online Community for people who find it hard to talk to those close to them.
You can also speak to one of the charity’s cancer support specialists on 0808 808 00 00 (7 days a week, 8am to 8pm).