Viagra is the brand name of the medicine Sildenafil. The safety of Sildenafil within six months of a stroke has not been determined; hence, one should not use it within six months of a stroke.
It is not clear as to why Neurobion Forte has been prescribed to you. Combinations of vitamin B-1, B-6 and B-12 have been found to be ineffective and are banned in all advanced countries. Even in India the combination was banned, but continued to be marketed due to loopholes in the law.