It's generally harmless but can cause permanent skin darkening.

一般而言，这种情况无碍健康，但会导致永久性的肤色变暗。

youdao

Almost 90 percent of women experience some form of skin darkening beginning in the second trimester.

几乎90%的女性在怀孕中期都会经历一些形式的皮肤变暗。

youdao

The brand-new melanin inhibiting ingredients highlight the whitening effect radically, prevent skin darkening, and relieve color spots; gradually brighten the skin tone, leave skin bright and fair.

全新抑黑成份从根本成因著手，加强重点美白效果，防止肤色变深，淡化色斑。日渐提亮肤色，令肌肤更明亮白皙。

youdao

"He had terrible headaches, the skin darkening, the weight loss,the weakness, the gastrointestinal complaints, the arthritis."

VOA: standard.2010.06.09

Latisse uses sterile wands for each application, but carries other risks, including eye redness and itchiness, and skin darkening, which goes away after the product is no longer used.

WSJ: Aches & Claims: Another Wand for When Mascara Isn't Enough

In fact, you may be knowingly willing to pay money to be exposed to this type of radiation in the name of darkening your skin.

FORBES: If Tanning Beds Were A Drug, They'd Be Illegal