You can deduct the interest on a home equity loan or a second mortgage. But — and this is a big but — only if you use the proceeds to substantially improve your house, and only if the loan, combined with your first mortgage, doesn’t add up to more than the magic number of $750,000 (or $1 million if the loans were existing as of Dec. 15, 2017).
If you use a home equity loan to pay medical bills, go to Paris, or for anything but home improvement, you can’t write off the interest on your taxes.