Giving breast cancer patients cough syrup might tell doctors how well they will respond to a vital drug.
Women often respond differently to Tamoxifen, which can mean they need a higher dose to get the same effect.
Researchers from the Erasmus Medical Centre in Rotterdam noticed the body deals with the active ingredient of cough syrup the same way - offering an easier way to make this calculation.
Charity Breakthrough Breast Cancer said it might cut side-effects for women.
Tamoxifen has helped improve UK breast cancer survival rates over the past two decades.
It works by blocking the effects of the female sex hormone oestrogen, which can stimulate growth in some tumours.
However, to work properly, the drug needs to be broken down, or metabolised, and some women appear better able to do this than others.
There is currently no easy way to tell in advance which women will be "good metabolisers" and which bad.
The Dutch researchers believe that the drug dextromethorphan, a drug which suppresses coughing, and often the active ingredient of cough syrup, could help.
It is broken down in exactly the same way as tamoxifen, and, is relatively harmless in comparison with the powerful anti-cancer drug.
They presented their work at a cancer treatment congress in Berlin.
Anne-Joy de Graan, who led the project, gave breast cancer patients a small dose of cough syrup before taking their Tamoxifen pills two hours later.
Blood samples were taken to see if the processing of the cough syrup drug matched that of the Tamoxifen.
The results showed that levels of the cough drug accurately predicted levels of the chemicals produced when tamoxifen is broken down.
One patient was taking an anti-depressant known to interfere with tamoxifen metabolism, and a corresponding effect was spotted in the dextromethorphan levels.
Anne-Joy de Graan said: "Tamoxifen is prescribed to women for as much as five years, so it is highly important to know beforehand if the therapy is going to be effective.
"When it is known that a woman metabolises tamoxifen poorly, a switch in drugs or an increase in dose can be considered."
She now plans to refine the test and study it in a larger group of patients.
Caitlin Palframan, from Breakthrough Breast Cancer said, that while it was early research, predicting a woman's response to tamoxifen was "an important question".
She said: "This kind of work could help us to improve a woman's chance of successful breast cancer treatment and spare her unnecessary side effects by giving her the right drugs at the right dose for her."