This means a significant boost to vaccine supply in the country even as Covid-19 cases continue to surge.
"At this time, given the public health emergency, FDA is advising that it is acceptable to use every full dose obtainable (the sixth, or possibly even a seventh) from each vial, pending resolution of the issue," the FDA said in a tweet on Wednesday.
Each Pfizer vial is supposed to have five doses of the vaccine.
"However, since the vials are preservative-free, it is critical to note that any further remaining product that does not constitute a full dose should not be pooled from multiple vials to create one," the FDA said.
According to a report in Politico, the extra vaccine doses could potentially expand the nation's supply by up to 40 per cent.
A Pfizer spokesperson told the USA Today that "the amount of vaccine remaining in the multidose vial after removal of 5 doses can vary, depending on the type of needles and syringes used."
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Pfizer said it cannot provide a recommendation on the use of the remaining amount.
The US FDA gave the emergency use authorisation for Pfizer-BioNTech Covid-19 vaccine last week. The US started its Covid-19 vaccine rollout nationwide on Monday as the death toll in the pandemic topped 300,000 in the country.