Drugs such as Glivec and Sutent reduce the size of tumours but could also make them more aggressive and mobile, it is claimed.
A little-studied group of cells called pericytes that provide structural support to blood vessels act as "gatekeepers" to pen in cancer, scientists have discovered.
Pericytes are wiped out by some advanced cancer drugs that prevent the growth of tumour-nourishing blood vessels, the research shows. As a result tumours find it easier to "metastasise", or spread around the body.
Tests on mice showed that both Glivec and Sutent depleted pericytes by 70% while metastasis rates tripled.
Glivec, the brand name of the drug imatinib, and Sutent (sunitinib) have both been shown in trials to increase patient survival by a significant degree.
However, the research raises the possibility that ultimately they might help cancers become more deadly. Metastasis to vital organs such as the liver or brain is the chief reason why people die from cancer.
The US scientists, whose work is reported in the journal Cancer Cell, began by removing pericytes from breast cancer tumours in genetically engineered mice.
They saw a 30% decrease in tumour volumes over 25 days, but also a three-fold increase in the number of secondary tumours growing in the animals' lungs.
Dr Joanna Owens, science information manager at Cancer Research UK, said: "This research helps us understand more about how they work in different patients, and refine clinical practice. These types of drugs are only used after extensive trials that demonstrate a clear benefit for some patients."