A Pfizer employee who spoke to ABC11 said it happened in a span of 60 to 90 seconds and there was no time to think. He said first the lights flickered inside the facility and then he said it sounded "like a bomb went off.
The worker said he and his group immediately ran to a designated safety area.
"I've got reports of 50,000 pallets of medicine that are strewn across the facility and damaged through the rain and the wind," he said.
Pfizer is one of the largest employers in the county.
Late Wednesday afternoon, the National Weather Service estimated that the twister was at least an EF-2 tornado with indicated winds of up to 135 mph.