My brother had undergone circumcision for which he was given anaesthesia on his back. after 20 days he still has severe headache. is this normal.

Spinal headache: Sounds like you're referring to epidural anesthesia (by your spelling, i assume you don't live in the states) which can be complicated by a spinal headache due to spinal fluid leaking out from puncture site. Check out http://www.Mayoclinic.Com/health/spinal-headaches/ds00643. Then report this to anesthesiologist who can confirm diagnosis & offer "blood patch" to plug leak & stop headache.

No : This sounds as though he has a post-dural puncture headache. This usually resolves within 10 days, but can last longer. I would go to the anesthesiologist who performed the spinal and ask for help.