Giving cough syrup to breast cancer patients might reveal on how well they will respond to a vital drug, researchers have revealed.
Women often respond differently to Tamoxifen, which can mean they need a higher dose to get the same effect, reports the BBC.
Researchers from the Erasmus Medical Centre in Rotterdam have found that 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 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 that 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.
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.
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."
The findings were presented at a cancer treatment congress in Berlin.