Have you tried cognitive therapy for your panic disorder and/or have your doctors looked into what may be causing this? The reason I ask is because there are many different reasons/causes for one to have panic disorder or to experience anxiety. There may be chemicals just a little bit off in your body or in my case, I had allergies that went undiagnosed for some time that made me feel anxious, rapid heartbeat and all the other terrible symptoms that made me nervous and panicky. You can do alot with a few little changes, ie your diet and behavior therapy. I have alot of compassion for you because I know how it feels and I just want to let you know that there are ways to help improve your condition. As for St. John's Wort, I was also told to try it. I had a bad reaction that made me feel spaced out and my heart raced so I'm not a big fan of it and neither are many doctors these days. If possible, try the natural approach first. You may see a noticable improvement. If you have any questions, I'll be happy to share with you whatever I know.
God Bless, gg