That may be the effect of the new, untried monetary medicine—negative interest rates—already in use by the European Central Bank, its continental counterparts in Scandinavia and Switzerland, and most recently by the Bank of Japan.

It’s too early to judge the efficacy of negative interest rates, which have been in effect only since 2015 in Europe and just a few months in Japan. But a Federal Reserve Bank of St. Louis economist casts doubt on the impact of negative rates, both in theory and in practice.

Rather than a subsidy for borrowers, negative rates act as a tax on lenders, writes Christopher J. Walker, director of research at the St. Louis Fed. The theory behind the novel policy is that, by charging a fee on banks’ reserves, they will be encouraged to lend out those funds and thus spur the economy.

In actuality, this cost amounts to a tax, he continues, and has to be borne by someone. Banks could absorb it in narrower profit margins. Or they could pass the cost on to borrowers in the form of higher interest rates or fees.

“None of this sounds very ‘stimulative’ for consumer spending. But then, no tax ever is,” Walker observes.

Bank stocks in Europe and Japan have taken hits after the imposition of negative rates in those regions, he goes on to note. Banks have been understandably reluctant to impose negative rates on depositors for fear of a backlash. Instead, mortgage rates have risen in Germany and Switzerland.

Even though the U.S. central bank had kept its key interest rate target above zero, and then enacted its first increase of this cycle in December, there are new signs that the Fed’s still-stimulative policy isn’t working as intended.

Each quarter, the Fed surveys senior loan officers at banks around the country for their subjective assessment of lending conditions—tighter, easier or about the same. In the first quarter, standards for business loans and commercial real estate loans tightened, although those for consumer and mortgage loans loosened slightly.

Indeed, writes JP Morgan Chase economist Daniel Silver, the latest quarter saw the most severe tightening of standards for commercial and industrial (C&I) loans as well as those for commercial real estate (CRE). At the same, demand for C&I and CRE credit eased as the economy slowed.

The slump in energy prices likely contributed to the tighter standards for C&I loans, he added, although the majority of banks said lending for oil and gas drilling or extraction accounted for less than their business-loan book. Tightening of CRE lending standards also may have been a response to regulators’ calls for prudence in this area.

In any case, increasing economic risk at a time of low interest rates leaves less of a margin of safety for lenders. In the capital markets, that was especially apparent with the widening of credit spreads in the high-yield bond market early in the year to compensate for increasing default risks.

The simplistic answer would appear for the Fed to raise interest rates to make lending more profitable. If it only were that easy.

The long-anticipated liftoff in rates by the Fed—at a time other central banks were easing policy, including through negative rates—helped to push the dollar higher through most of last year. In turn, commodity prices—especially oil—tumbled.

China’s central bank also followed the Fed’s tightening by keeping its currency pegged to the dollar, resulting in China having the highest real interest rates in the globe. The result, you’ll recall, was the sudden devaluation and plunge in China’s stock market late last summer, which put further pressure on commodities.

Expectations of as many as four Fed rate increases in 2016 sent global financial markets tumbling early this year. But the turnaround starting in mid-February—resulting in large part from reduced Fed rate-hike expectations—has followed a sharp unwind of a portion of the greenback’s rally. And in the process, crude oil rallied about 70%, from the mid-$20 a barrel range to the mid-$40s.

In addition, ECB President Mario Draghi hinted strongly in March that its rates won’t be lowered further into negative territory. And to get away from the “tax” resulting from negative rates, the ECB has proposed direct subsidies to banks that actually make loans.

As for the BOJ, its negative rate announcement in late January backfired by sending the yen soaring and stocks plunging, exactly the opposite of the expected response. And when the BOJ recently failed to lower rates further into minus territory, the yen jumped and the stocks slid again.

Confusing, to say the least.

The only certainty is that negative interest rates are an experimental medicine with uncertain side-effects. An estimated $9.9 trillion worth of global government bonds yield less than zero, which means investors—including pension funds and insurance companies—are paying governments to hold their money. And to remove the traditional alternative as a store of value—money stuffed in a mattress—the ECB said Wednesday it will phase out the 500-euro note (worth $575) starting in 2018.

Negative interest rates still undermine the basic notion of investing: savers forego current consumption and lend surplus funds in hopes of a future return; that in turn provides the capital needed by businesses to expand production and employment. As the St. Louis Fed points out, placing a tax on lending in the form of negative rates restrains lending instead of stimulating it.