Pfizer Inc. is forging ahead in its quest to sell an over-the-counter version of blockbuster cholesterol pill Lipitor, hoping to overcome skepticism that consumers can take the drug appropriately without doctor guidance.

New uncertainty surrounds Pfizer's prospects because of recent changes in clinical guidelines for treating people with cholesterol-lowering statin drugs such as Lipitor. The recommendations eschew familiar and relatively easy-to-understand targets for cholesterol levels in favor of a more complex assessment...