Commonly prescribed steroids could interfere with cancer treatments, warns new research.
It may explain why some patients do not respond well to immunotherapy drugs that destroy tumours.
Taking glucocorticoids (GCs) fuels a protein linked to poorer outcomes, say scientists.
Lead author Dr Tobias Janowitz, of Cold Spring Harbour Laboratory, Long Island, New York, explained: "GCs are very powerful suppressors of immunity and are consequently used to treat autoimmunity, which is when the immune system attacks healthy cells.
"We have previously shown that GCs can also break cancer immunotherapy. Now, here is perhaps a clue into how they are doing it.”"
Asthma, arthritis and eczema patients are all routinely prescribed steroids.
Immunotherapy uses the body's own immune system to recognise cancer cells - and kill them.
It can be administered as tablets, injections or inhaled. But patients sometimes experience side effects GKs can treat.
But they also produce a protein called CyC (Cystatin C), higher levels of which interfere with immunotherapy.
The finding is based on an analysis of data from the UK biobank, which holds detailed genetic and health information on around half a million Britons.
Cancer patients who were more likely to produce CyC in response to GCs had a worse overall survival rate.
These patients were also less likely to benefit from treatment - suggesting CyC production within a tumour may contribute to the failure of immunotherapy.
The discovery was confirmed by experiments in mice. When a CyC-producing gene was deleted so it was no longer present in tumours, they grew slower.
Co author Dr Hannah Meyer, also from Cold Harbour, said: "It is really powerful to come at this from multiple angles and support the findings through many approaches.
"Clever genetic models gave us some indication of which experiments to design to help us answer the question of what this molecule does."
Prof Janowitz plans to continue studying CyC. He hopes this could greatly help future patients.
He added: "The research has given me an impetus to find out more about the function of this molecule, specifically in the context of cancer immunotherapy.
"Perhaps its function can be targeted to enhance the success of cancer immunotherapy."
The study is published in the journal Cell Genomics.
