Side-effects of GBS medication if incorrectly diagnosed

My dad was admitted to hospital and was discharged on Friday 21 July 2006. The doctors were not sure what kind of illness he had. He had a white lung. He was not very ill and had no fever, but he was treated for pneumonia and discharged. On Monday 24 July 06 he had a num sensation in his feet. I took him to hospital the next day and was seen by a neurologist. Test were done, blood was taken, and we were told to come back on 27 July 06 for the results. On 27 July 06 we went to find out what the results were. They told us they need more blood and a LP for more tests and then we may go. On 4 August 06 he was admitted to hospital, by then he was paralysed for the waist down. He had no controll and was in nappies. On Saturday 5 Aug 06 he was diagnosed with GBS. Treament started, on Monday morning 7 Aug 06 he was delirious. Tuesday morning, 8 Aug 06, the neurologist called me, she needed to speak to the family. My dad was diagnosed with lung cancer and spinal cord (marrow) cancer. We were also told that the spinal cord cancer is very, very aggressive and its in the brain by now. (Delirious come) My dad died on 10 August 06. I need to know what the side-effects of GBS medication are if incorrectly diagnosed.