If the debt had no connection to business activity or purpose, then it would be considered a personal bad debt. Usually this is a result of making a loan to a friend or relative who fails to repay the loan. This type of bad debt is considered a short-term capital loss and is claimed on Schedule D of the Form 1040. As such, it will first be matched with the capital gains on the Schedule D. Any remaining loss after matching against capital gains will then flow from the Schedule D to the front of the Form 1040, with a limit of $3000.00 per year ($1500.00 for married filing separately) in deductible capital losses. The balance will be carried forward to future years and reported as a carry forward on the next year's Schedule D.
Personal bad debts must be carefully documented and this is especially important if the bad debt was to a relative. Under audit, the IRS will request a copy of the promissory note or other proof that the loan was really a loan and not intended to be a gift.
For both personal and business bad debts, but again especially for personal bad debts (and add emphasis to that if the loan is to a relative), it is crucial to have careful documentation on your efforts to collect the debt. Normally this includes copies of demand letters and other collection correspondence. It would also include the services of an attorney or filing a collection lawsuit. If you make no effort to collect a personal bad debt, then in general you may not write it off the bad debt on your tax return.