It seems like the IRS has you coming and going on Social Security. While you are working for a living, you must pay taxes into the system to provide benefits for current retirees. Then, when you finally retire, you're entitled to receive retirement benefits but they might be subject to tax as well.

Don't confuse the two taxes. The Social Security tax you pay as an employee is a payroll tax that applies to wages, commissions, and other compensation as part of the FICA tax. An employee's combined FICA rate for Social Security and Medicare in 2017 is 7.65% on the first $127,200 of compensation and 1.45% (Medicare only) above that. But the tax that may apply to Social Security benefits you get in retirement is a federal income tax that is reported along with other items on Form 1040. It's more complicated than the payroll tax.

Here's how it works: You're liable for tax on Social Security benefits if your provisional income (PI) exceeds certain thresholds in the tax law. For this purpose, PI is the total of (1) your adjusted gross income (AGI), (2) your tax-exempt interest income (for example, from municipal bonds), and (3) one-half of the Social Security benefits you received. For example, if the combined AGI of you and your spouse is $100,000 and you collect $5,000 in municipal bond income and $20,000 in Social Security benefits, your PI is $125,000 ($100,000 + $5,000 + $20,000).

There are actually two thresholds for computing the tax on Social Security benefits.

Threshold 1: For a PI between $32,000 and $44,000 ($25,000 and $34,000 for single filers), you're taxed on the lesser of one-half of your benefits or 50% of the amount by which PI exceeds $32,000 ($25,000 for single filers).

Threshold 2. For a PI greater than $44,000 ($34,000 for single filers), you're taxed on 85% of the amount by which PI exceeds $44,000 ($34,000 for single filers) plus the lesser of the amount determined under the first tier or $6,000 ($4,500 for single filers). Silver lining: You'll never owe tax on more than 85% of your total benefits.

These two thresholds aren't indexed annually for inflation. If your PI exceeds a relatively low level of $32,000 ($25,000 for single filers), you'll owe the tax year in and year out. And you'll get hit with the higher tax rate every year that your PI exceeds just $44,000 ($34,000 for single filers).

What can you do about it? You might lower your PI by harvesting capital losses to offset capital gains or deferring taxable income to the following year. But remember that the income from tax-free municipal bonds counts against you in the calculation of PI. Consider all the relevant factors, including the potential tax implications for Social Security benefits, in your investment decisions.