Valerian root tea or powdered tablets are probably the most effective OTC stuff you could take to bring you down a notch. Otherwise, the occasional Benedryl may be worth a shot as soon as you feel one coming on, but the effects will likely wear off after repeated use. Watch out for alcohol or medications not prescribed to you, these may make matters worse.
Is there a trigger in your work environment that brings a panic on? Maybe you should try to determine what's bringing them about.