These are difficult times for tax preparers. Changes in Treasury Department rules have made it an especially tricky tax season, with lower refunds taking many Americans by surprise. That’s led preparers to stock up on chocolate and painkillers—both for unhappy clients and for themselves.

John Dundon, a preparer in the Denver area, says he bought tissues by the box last year for emotional clients. This year, he bought a carton of 24 boxes that clients are going through rapidly.

He recently advised a couple in their 70s that although their overall tax rate dropped from 21% to 18% for 2018, they owe the IRS $2,000 this year. They had been expecting a refund of the same amount. When Mr. Dundon delivered the bad news, the wife pulled her hair, waved her arms and yelled, “This is unacceptable!”—“like it was my fault,” he says.

Mr. Dundon’s wife brews beer as a hobby, and he’s grateful for a glass of it at the end of his workdays, which now stretch to 9 p.m.

In December 2017, Congress passed a tax overhaul that lowered income taxes for an estimated 65% of taxpayers and raised them for 6%, according to the Tax Policy Center, a Washington research group. The remaining 29% saw their taxes unchanged.