People are owed more than £2.6 billion in IOUs from friends, family and colleagues, a new report suggested.
Research by O2 Money showed that almost half of adults lent money to someone last year, mainly to friends, children or work colleagues.
One in two people who lent money were still waiting to get it back, while the average repayment time was 60 days.
Cash was lent to people who said they didn't have money on them or who had left a credit card at home, while they often didn't like to ask for the money back because they felt awkward.
Welsh people loaned the least (an average of £71 last year) but were the quickest to pay back an IOU, taking around 36 days, while over 55s were the most generous age group, according to the study among 2,000 adults.
James Le Brocq, Managing Director of O2 Money, said: "Outstanding IOUs can be an expensive business. They are a source of embarrassment and it's a topic of conversation many of us are reluctant to raise."