Of the estimated 83,000 cancer carers in Scotland, 26% carry out duties such as change dressings and help manage infections.
Across the UK more than one in five carers - around 240,000 people - carry out these tasks. 53% say they have had little or no instructions or training from a health professional.
One in five cancer carers who have had some advice or training said it was not enough.
The survey of more than 2000 carers in the UK, for the charity Macmillan Cancer Support, found 63% of those with no training or whose training was not good enough had been left feeling distressed, and 50% said the situation left them feeling frightened.
One in three said they worried their loved one might need to go into hospital and one in nine said this had actually happened.
Of those who perform health-care tasks, 36% have had to call a doctor urgently or call 999 to get support or advice on how to help the person they care for.
As well as those carrying out health-care tasks, the research revealed more than 40% of cancer carers in Scotland felt apprehensive when they started the role, 40% felt unprepared, 12% felt isolated and 7% felt abandoned.
Allan Cowie, Macmillan's general manager for Scotland, said: "Thousands of people across Scotland give up hours of their time to look after loved ones going through cancer and it is vital these people are supported.
"It is unfair to expect carers, who are unprepared or not supported, to carry out tasks which clearly require training. It's also important assumptions are not made about carers taking on these tasks - explicit consent from carers must always be sought.
"While we champion people affected by cancer who wish to have more control over their own care, we recognise this must go hand in hand with support being available to patients and carers when they need it.
"No-one should be placed in a situation of having to provide care or tasks as an unplanned consequence of treatment as this study implies."
The charity is now calling for the issue to be recognised by the Scottish Government in response to the consultation on carers announced last week by the First Minister. It was recently announced that Macmillan and the Scottish Government were working together on a project to transform care after treatment for cancer patients, and the charity will suggest the needs of carers could be addressed as part of this.
In June cancer charities were among groups that called for more support for people in caring roles after a survey that suggested 75% of carers were unprepared for the impact of the role.
The report from Carers Week said they were being "woefully let down by a lack of support".
The charities urged GPs, health and social care staff and the Scottish Government to ensure more support was given from day one.
That survey of more than 2100 carers found 81% said they were not aware of the support available. 35% believed they were given the wrong advice about support on offer.
It also found carers often struggled to balance work and caring responsibilities, with 45% saying they had to give up work.