In December, pharmacists made the happy discovery that they could squeeze an extra vaccine dose out of Pfizer vials that were supposed to contain only five.

Now, it appears, the bill is due. Pfizer plans to count the surprise sixth dose toward its previous commitment of 200 million doses of Covid vaccine by the end of July and therefore will be providing fewer vials than once expected for the United States.

And yet, pharmacists at some vaccination sites say they are still struggling to reliably extract the extra doses, which require the use of a specialty syringe.

“Now there’s more pressure to make sure that you get that sixth dose out,” said Michael Ganio, the senior director for pharmacy practice and quality at the American Society of Health-System Pharmacists.