Almost a third of new retirees will see their health deteriorate during the first five years of retirement, according to insurer LV=.
Its survey revealed that 31% of respondents said their health had worsened, and 10% said they had been diagnosed with a serious illness during the early years of their retirement.
Health issues aside, the survey also exposed other life changes that can affect the financial circumstances of those giving up work.
For example, 26% of respondents had given financial help to family members, 19% had moved home and 24% had carried out major work to their existing home.
There were happy things that those surveyed experienced too - 38% welcomed a new grandchild, 10% received an inheritance and 2% moved abroad.
Vanessa Owen, head of annuities at LV=, said: "There are thousands of people coming up to retirement every week, entering the unknown and trying to transition from working to retired life, and many of these people will find themselves in a very different set of circumstances five years down the line. At that point it could be too late to change the way they can access their retirement funds.
"It is essential that people do their homework and don't just plump for the annuity offered to them by the company they have saved a pension with. We would always recommend that people seek specialist independent financial advice, to consider all the options available, from a lifetime annuity, enhanced annuity, fixed-term annuity, investment-linked annuity or income drawdown.
"There are lots of different ways to structure retirement income but many won't be aware of these options unless they seek advice."
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