The nation's largest drugmakers have paid at least $8 billion in fines for repeatedly defrauding Medicare and Medicaid over the past decade, but they remain in business with the federal government because they are often the sole suppliers of critical products, records show. Pfizer, the maker of drugs that help alleviate arthritis and other ailments, has paid almost $3 billion in fines since 2002 and entered into three corporate integrity agreements with the Department of Health and Human Services aimed at preventing future fraud. It and other companies are fighting attempts by Congress to exclude them from government business because of their history of fraud.