The current practice to determine the combined geosynthetic strength reduction due to installation damage and creep is to multiply the individual strength reduction factors for damage and creep together. However, it is difficult to know if this procedure results in conservative design, nonconservative design, or is sufficiently accurate for practical purposes. The effect of synergism on the combined effect of polymeric creep and installation damage is explored in the paper by reviewing published and unpublished constant load (creep) data for both undamaged and installation-damaged geosynthetic specimens. A methodology is developed that uses data from both constant load (creep) tests and index tests to reconstruct installation-damaged isochronous creep curves. Based on the limited data available and the interpretation of reconstructed isochronous curves, it is shown that multiplication of creep reduction and installation damage factors may be conservative and hence results in errors on the safe side for design.