Carers looking after somebody with cancer deliver injections and manage catheters often without any training, a poll has found.
More than one in five carers - around 240,000 people - do things such as change dressings and manage infections, of whom 53% say they have had little or no instructions or training from a health professional.
One in five (21%) cancer carers who have received some advice or training said it was not enough.
The survey of more than 2,000 carers, for 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 (34%) said they worried their loved one may need to go into hospital and one in nine said this had actually happened.
Of those who perform healthcare tasks, 36% have had to urgently call a doctor or 999 to get support or advice on how to help the person they care for.
Macmillan is now calling for changes in the care bill to ensure the NHS in England supports cancer carers.
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Ciaran Devane, chief executive of the charity, said: "Not only do cancer carers give hours of emotional support and practical help, they are performing clinical duties.
"It's a huge responsibility they take on out of duty and love. Families and carers are the backbone of society and they deserve to be supported.
"Without support, cancer carers can go beyond breaking point which is bad for them and their loved one but is also costly to the NHS and ultimately to the taxpayer.
"By identifying cancer carers and explaining what information and support is available, health professionals can vastly improve their quality of life and help them to continue caring - which is what they want to do."
Mr Devane said it was "nonsensical" that the care bill as it currently stands places a legal duty solely on local authorities to identify and support cancer carers.
"It's actually the health service that has the most contact with them," he added.
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Pamela Digney, from Lincolnshire, looks after her husband Roy, 75, who had cancer removed from his spine.
She said: "My husband is paralysed from the waist down from his operation, so I have to help him with everything.
"I have to administer morphine patches and liquid morphine for pain relief, as well as help him with his catheter. Infection control is also a constant concern.
"I haven't been given adequate training or information to help with these things, and it leaves you feeling quite vulnerable when you have to do them on your own."
Care and Support Minister Norman Lamb said: "Carers make a huge contribution to society and we want to do all we can to support them."
"We agree that there needs to be better joint working and proposals already in the Care Bill will mean that local authorities will have to co-operate and work closely with the NHS to identify and support carers.
"We have also provided £400 million to the NHS for carers breaks and given over £1.5 million of funding to help develop initiatives with GPs, nurses and carers organisations to train people to help support them in their caring roles."