Nike said it was "saddened and surprised" at her admission that she tested positive for a banned substance at the Australian Open in January.
Ms Sharapova said she had been taking meldonium since 2006, on the advice of her family doctor.
She is one of the highest paid female athletes with earnings of over $30m last year from winnings and endorsements.
In addition to the moves from Nike and Swiss watchmaker Tag Heuer, German carmaker Porsche said it was "postponing planned activities" with Ms Sharapova until the situation became clearer.
Nigel Currie, an independent sports consultant, said brands would not have responded so quickly five years ago, but they now have to react faster in the modern, social media environment.
"They are paranoid about their image, and the slightest risk to their image, they run to the hills."
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