In the process of finding a breast cancer drug, doctors have identified a main ingredient used in cough syrup – tamoxifen.

"This study is starting to identify a personalized medicine type of approach that will help identify those patients who should have their tamoxifen doses adjusted," said Dr. Leonidas Koniaris, who's familiar with the study findings.

The authors of the study said it is tough at the moment to set a proper dose of the drug, as it may cause numerous side effects if the dose is off. They are now focused on finding if dextromethorphan, a major ingredient in cough medicine, is metabolized by the body at about the same rate as tamoxifen.

Dextromethorphan "is a so-called 'probe' drug, a harmless substance that can be used to predict the metabolism of another drug," de Graan said in a news release provided by the European Cancer Organization.

Researchers gave 30 milligrams of dextromethorphan to about 40 patients and gave tamoxifen about two hours later. They observed the metabolism rates of both the drugs, and found similar results with both.

However, researchers conclude the cough medicine requires more testing to be used in treatment of breast cancer. For now, they noted, "it certainly looks quite promising."