@BTI
I'm hesitant to answer because I don't know what you mean by incurring expenses on behalf of a business.
A typical non-business bad debt would be something like you hire a contractor to remodel your kitchen, pay a deposit, then they skip town. You have a contract that created a legal obligation and you aren't getting what you paid for.
If you are in business, you deduct your expenses as per usual. You don't get an extra deduction for a contract that goes bad, your tax "Reduction" comes from the fact that you don't have income to offset the expense, and when your income is lower, your taxable profit is lower. For example, you are a painter and you have a contract to purchase some speciality paint for a specific job that can't really be used for other jobs. The client goes out of business and you are stuck with the specialty paint. You already deducted the paint as a business expense, now you don't get income, so your taxable profit is lower, but you don't get a second, extra deduction.
So it depends on what you mean by incurring expenses for another business that were not reimbursed. Was this in the course of your own business activities, or your non-business personal life? How would you incur expenses, and what kind of contract or legal obligation does this create between you and the business?
This is what the IRS says
https://www.irs.gov/taxtopics/tc453
