hi. i've been smoking for the last 10 years or so, and at first paranoia attacked me so badly that even thought that i was going to die of a heart attack.
In my case it was because all that misinformation about cannabis use. I was told since a kid that only bad people (murderers, thieves) used cannabis and that if you smoked a joint you'd end up prostituting yourself to get your next fix and living in the streets.
all that terrorism was causing me a bad trip.
There is physical reality though and is that your heart rate changes during cannabis use. Dont be afraid of it. if you feel its too fast or too slow, just breathe and calm yourself. You are not the first guy feeling that.
Use your will power to make the drug hallucination a beautiful peaceful trip. If you believe in some sort of god, ask him or her to accompany you on your trip.
I think cannabis is like a magnifying glass. If you feel anxious, nervous or scared, that feeling grows and you get to know hell. But if you relax and focus on good things, it becomes a beautiful, really beautiful experience.
That, and check the quality of the cannabis you are buying. If it smells other than plant-like, dont use it.