Counterfeit versions of the cancer drug Avastin obtained by European regulators contain a variety of chemicals but not the active ingredient found in the genuine drug, according to drugmaker Roche.

The Swiss company said in a statement that an analysis of the contents of the vials picked up traces of 10 substances, ranging from starch and salt to solvent chemicals such as acetone, which is used in paint thinner.

Medical experts said Tuesday that the chemicals are unlikely to harm patients, provided they are not taken in large doses.

"They're not great to have in your system, but depending on the concentration your body can probably handle them pretty well," said Dr. Miguel Fernandez of the South Texas Poison Center in San Antonio. "It's the dose that makes the poison."

Roche did not provide details on the level of the chemicals found in the vials. The company said none of the fake products contain the active ingredient found in Avastin, a protein-based drug infused at hospitals and doctors' offices.

"The counterfeit product is not safe or effective and should not be used," the company said in a statement.

A Roche spokesperson said Wednesday that the company is not aware of any counterfeit Avastin distributed in Canada.

Avastin is used to treat cancers of the colon, lung, kidney and brain. The drug is one of the most widely used cancer drugs in the world, generating about $6 billion US a year in sales.

The U.S. Food and Drug Administration is investigating the source of counterfeit Avastin imported and distributed by a U.S. wholesaler to doctors here.

British regulators have confirmed that 41 vials of fake Avastin were shipped to the U.S. Five have been recovered while 36 which are still missing. Authorities in Europe have traced the counterfeit product back through distributors in Britain, Denmark and Switzerland. The original country of origin is still unclear.

Counterfeit incidence on the rise

Dr. Philip Cole of Johns Hopkins University said several of the substances appear to be common to biotech drugs, including starch, salt and benzoic acid.

"But I don't think any of them are useful for getting the functional effect of Avastin," said Cole, who directs the pharmacology program at Johns Hopkins School of Medicine.

Cole said the presence of the solvent acetone was concerning, since it does not have a medical use.

"We do make some acetone in our bodies as a byproduct of various metabolic pathways," Cole said. "Small quantities are not dangerous, but it certainly can be toxic at higher levels."

Experts say gauging harm from a counterfeit cancer treatment is very difficult because drug infusions are spaced out over weeks and months. A colon cancer patient, for example, might receive 18 to 20 Avastin infusions over six months. Missing one dose seems unlikely to have a dramatic effect on survival odds, but it's not provable either way.

Incidents of counterfeiting reported by drugmakers have increased steadily over the decade to more than 1,700 worldwide last year, though only 6 percent of those were in the U.S. The rise in counterfeiting comes as pharmaceutical supply chains increasingly stretch across continents. More than 80 per cent of the active ingredients used in U.S. pharmaceuticals are now manufactured overseas, according to a recent congressional report.

Roche sells Avastin in 120 countries and manufacturers and packages the drug at eight sites worldwide.