Muscle damage related to the use of statins is not uncommon, but now researchers say tissue classified as injured by the cholesterol-lowering drugs can't always be detected through a standard blood test.

Studies suggest 10 to 15 per cent of people taking statins experience some kind of muscle pain or weakness as a side effect of the treatment. A smaller but unknown percentage have stronger, persistent pain, called myopathy.

If doctors suspect statins are causing pain, they usually test for an enzyme in the blood known as creatin phosphokinase (CPK), which leaks from damaged muscles.

But a study by U.S. and Swiss researchers published in the July 7 issue of the Canadian Medical Association Journal found muscle damage even when tests came back negative for elevated levels of serum CPK.

The researchers looked at leg muscle biopsies from 44 people who had "clinically diagnosed statin-associated myopathy" and were either currently taking statins or had recently taken them.

They found that 25 of those patients also had muscle injury, in spite of normal blood tests. Only one patient with "structural injury" had a circulating level of creatine phosphokinase that was elevated more than the upper limit of normal, the researchers said.

They classified muscles as injured if two per cent or more of the muscle fibres in a biopsy sample showed damage.

Biopsies of some of those patients revealed damage to muscle fibers that could continue even after they stopped taking the medications.

Statins are one of the most widely prescribed medications in the world, used to reduce the risk of cardiovascular disease, and are sold under such brand names as Lipitor, Crestor, Zocor, Mevacor, Lescol and Pravachol.

Last year, Canadians filled 26 million prescriptions for the drugs, worth $2.2 billion.