Abstract: A medicament vial safety cap which may be formed integrally at the time of manufacture of the medicament vial or added as an accessory to an existing conventional medicament vial allows an equalization of pressure to occur within a chamber adjacent to the stopper of the vial so that aerosolization of medicaments does not occur as a hypodermic needle is inserted or withdrawn from the vial. A hydrophobic filter, communicating between the vial safety chamber and the ambient, allows the pressure equalization to occur while trapping molecules of medicament inside the chamber.