Let R be a simple algebra over its extended centroid C, and let f(X1,…,Xn) be a noncommutative polynomial having zero constant term. We denote by f(R)+ the additive subgroup of R generated by all elements f(x1,…,xn) for xi∈R. It is proved that if charR=0, then f(R)+ is equal to {0}, C, [R,R], or R. As to the case charR=p>0, an example of a polynomial f satisfying [R,R]⊊f(R)+⊊R is given. Also, the polynomials f with f(R)+=[R,R] are characterized if charR=0 and R≠[R,R]. Moreover, we work on the context of centrally closed prime algebras to get more general results.