A {\it new} formula for the force vs extension relation is derived from the
discrete version of the so called {\it worm like chain} model. This formula
correctly fits some recent experimental data on polymer stretching and some
numerical simulations with pairwise repulsive potentials. For a more realistic
Lennard-Jones potential the agreement with simulations is found to be good when
the temperature is above the θ temperature. For lower temperatures a
plateau emerges, as predicted by some recent experimental and theoretical
results, and our formula gives good results only in the high force regime. We
briefly discuss how other kinds of self-interactions are expected to affect the
elasticity of the polymer.Comment: 8 pages, 10 figure