I had a similar experience the first time I smoked. I had never done any mind-altering substance and I smoked 2 GRAMS of sativa alone. The resulting experience made me consider suicide multiple times: racing heart, altered perceptions, broken-record thoughts, shaking, extreme anxiety and depression, along with the guilt that I could have lead myself to psychosis by trying to have some fun. My consciousness bobbed in and out, I felt as if I was being drowned/knocked out and awoken multiple times every minute. After the experience, for something like 6 months, I too experienced unprecedented panic attacks and what is called Depersonalization Disorder, a chronic (no pun intended :P) sense of disattachment.

Because of the brain's plasticity, the more you do something, the more easily the brain repeats that neuronal pattern. This can be good when you are learning a new skill; but if you smoke and it ends in anxiety the brain can create a kind of "shortcut" to anxiety. Any stimuli, particularly the smell of weed can trigger this similar response. Similarly, I have seen heavy DXM users vomit at the sight/smell of cough syrup.

I enjoy smoking myself but oftentimes, being an anxiety-ridden person, smoking too much sativa can send me into chemical hell until I calm down.

Needless to say, I am still enjoying marijuana today. I would recommend trying a FEW hits of weed. If you drink alcohol, that can definitely take the edge off if you are an anxious smoker.

Weed is rarely unknowingly laced - coke/pcp are expensive. Who would waste them on an unsuspecting smoker?

Good luck on your journey.

-Fnatic.