Vaping, already linked to lung damage, may also have harmful psychological effects, a new study suggests. The researchers found a strong association between vaping and depression in a study of nearly 900,000 U.S. adults.

The apparent culprit: nicotine.

"There is a potential risk between e-cigarette use and depression," said lead researcher Dr. Olufunmilayo Obisesan. "E-cigarettes are not as harmless as people once thought they were," Obisesan added.

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"Because e-cigarettes contain nicotine, there could also be an association with depression," Obisesan said.