This emerged in new research by Nationwide Building Society.
The poll, which surveyed more than 2,000 people who are self-employed or receive statutory sick pay, highlighted that 59% of them had gone to work when ill or injured.
Furthermore, 32% admitted not following their doctor's advice because they couldn't afford to take time off.
And 43% would put off going to the doctors due to financial concerns - even if worried they may have a serious illness.
Nationwide's research highlights that, while 49% say they would benefit from a policy which would cover their income if off work for an extended period, just 27% actually have any income-protection cover in place - with 32% unaware of what such a policy is.
Money worries can have a negative impact on people's mental wellbeing, with 64% saying they fret about how they would cope financially if they needed to take four weeks or more off work due to poor health.
Three in 10 of those surveyed have nothing in place to support them financially should they be ill or injured.
And 29% would rely on statutory sick pay, which at £96.35 per week for up to 28 weeks is much less than many people need to cover the rising cost of living.
If on long-term sick and on a reduced income, many would use their existing savings (45%), make reduced payments (33%), or borrow money from family or friends (25%) or use a credit card or loan (15%).
However, during the pandemic 34% have already dipped into their savings - meaning they may now have less to fall back on.
As a result, 55% admit they could only survive for three months, while 27% would struggle after just one month.
As well as the immediate impact of long-term sickness, many people are also concerned about the longer-term financial impacts - with 49% saying they worry about the impact on their ability to get credit in future. This is particularly an issue for the self-employed, where 43% fear losing clients and 36% are concerned that their business would have to fold.
Jason Hurwood, director of protection, investments and insurance at Nationwide Building Society, said:
"When you are off work due to an illness or injury, worries about how you are going to pay your bills can make an already-stressful situation worse.
"So much so, that many are finding themselves in the very difficult position of having to put the need to earn money over their health by continuing to go to work, even when advised not to by a doctor. Worse still, financial concerns mean some are avoiding seeing their GP altogether, even when concerned they may have a serious illness.
"With no money put aside, many would fall into financial difficulty straight away and most would start feeling the strain after just three months. Very few opt for income-protection insurance, particularly the self-employed and those who get little or no sick pay from their employers, because of concerns about the cost or even just a lack of awareness that it exists."
Based on tax legislation at the time of publication. Please be aware that there will have been changes since this was published. Speak to your adviser for the most up to date information.
