How bad is it for people with nervous tension to have coffee? I am suffering from nervous tension and anxiety. Doctors have prescribed me regular anxiety reliever drugs and also asked me to completely stay away from tea, coffee, cola or any other beverages having caffeine.

I don't heed to their advice. But I do feel tense if I have coffee especially in excessive dose. However I am addicted to coffee.

Now why coffee is bad for me and how much caffeine per day is safe for me?
Caffeine effects everyone differently, both physiologically and psychologically. If your doctor(s) say to avoid caffeine, I would heed their advice. 

Getting off caffeine can be uncomfortable but totally possible - start to reduce the amount you have every day (e.g. switch from regular coffee to regular tea to decaf coffee to nothing), and you will minimize headaches. It will take time to readjust to normal (2-4 weeks for me, but again, everyone is different), but it will happen, and you will notice you feel less anxious and more calm and steady.