The phased polaron beam is a type of directed-energy weapon used in Dominion warships and sidearms.
When phased polaron weapons were first encountered by Starfleet in late 2370, they were able to penetrate standard deflector shields with ease. (DS9: "The Jem'Hadar") By the time of the Battle of the Omarion Nebula the following year, the Defiant's shields were modifed to repel the Jem'Hadar's phased polaron beams. However, it seemed that the Romulan and Cardassian ships did not have the same modifications. (DS9: "The Die is Cast")
In 2372, while on a diplomatic mission to negotiate a trade agreement between the Karemma and the Ferengi, the Defiant was attacked, and again repelled enemy fire. (DS9: "Starship Down") The Dominion did not become aware of the shield modifications until the Second Battle of Deep Space Nine in 2373, when Dominion officers were surprised to see Deep Space 9's shields holding. (DS9: "Call to Arms").
Though much effectiveness to Federation Shields was eliminated, the Phased Polaron Beam continued to be a powerful and deadly weapon, to most any race, save for the Borg.
Jem'Hadar polaron rifles include an anticoagulant in the energy stream. This prevents any non-fatal rifle wounds from healing, forcing the enemy to devote resources into caring for them. (DS9: "The Ship", "Change of Heart") These weapons were replaced by plasma rifles in late 2375. (DS9: "Tacking Into the Wind")