PHILADELPHIA – Federal prosecutors accused the nation’s biggest pharmacy benefit-management company Monday of defrauding clients by destroying prescriptions, switching patients’ medications without their consent and giving favorable treatment to drug companies that paid hefty rebates.

A lawsuit filed in Philadelphia accuses Medco Health Solutions of altering prescription records to avoid paying late penalties in its mail-order business and steering customers to drugs made by its former parent company, Merck, instead of competitors’ less-expensive products.

Medco officials immediately called the charges either false or overstated.

“The full story will show that our people are highly skilled, our policies are rigorously enforced and our pharmacy practices, which are regularly inspected by state boards of pharmacy, lead our industry in lowering the cost of providing high-quality health care for millions of Americans,” the company’s chairman, president and CEO, David Snow, said in a statement.

The company acknowledged that some violations cited in the complaint occurred, but said they were isolated incidents that happened years ago and have since been corrected.

Pharmacy benefit-management companies use bulk-purchasing power to lower drug costs.