Once you pull money out of your plan, those dollars no longer benefit from long-term market returns.
If you have a pool of emergency funds, it's best to use that money first. If you're managing debt, it's even better to build that repayment into your budget.
Even your boss wants you to keep your hands off your retirement plan savings.
That said, here are three extreme cases that may warrant a 401(k) loan.
You have an immediate emergency. "Say that you need to meet the deductible on your high-deductible health-care plan, and you have no money in your health savings account," said Aaron Pottichen, president of retirement services at CLS Partners in Austin, Texas.
He is referring to the tax-advantaged savings account that individuals may use to cover qualified medical expenses. It's also known as an HSA.
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You have an urgent cash need, but your credit precludes you from obtaining a competitive interest rate. Ask yourself what you can repay in five years.
"From an interest-rate standpoint, [a 401(k) loan] can help, but be careful not to max out your term," said Tyler Harrison, managing member of Efficient Plan in Denton, Texas.
You need to pay off high-interest debt that's hampering your long-term financial goals. This is the case if the interest rate on your 401(k) is lower than what your creditor is offering you.
"If you're in 'pay down debt mode,' it's all about what's your cheapest interest rate and how fast can you get the debt down," said Pottichen.