Hello Joe S,   Both Lipitor and Zocor are members of the family of medications called statins.  Statins work by inhibiting an enzyme in your body that makes cholesterol from ingested and body fats.   Both drugs have similar side effect profiles.  You have to watch out for muscle aches, and your doctor should monitor your liver with blood tests, since statin drugs can raise liver enzymes.  Though extremely rare, both statins can cause a serious side effect related to muscle breakdown called rhabdomyolysis.   In all, the side effect profile is nearly identical for these two drugs, especially considering both of the dosages you mention are essentially the lowest recommended doses.   To your health, Neil MD