Is light therapy safe if you have or have had cancer?

The short answer

Search for red light and cancer and you find two stories that contradict each other. One says light can make cancer cells grow. The other says light therapy is given to cancer patients in hospitals.

Both are true. The difference lies in where the evidence comes from: the worry comes almost entirely from cell cultures, the reassurance from people.

How much research is there on light and cancer?

The largest safety review in this area gathered 57 studies. The full text says how they break down, and that is the most important figure in this article.

Look at that split. Forty-one cell culture studies against seven trials in humans. If you read online that red light makes cancer cells grow, you are almost certainly reading about one of those forty-one. That is not deception. It is simply a different kind of evidence from the kind you base a decision about your body on.

Why do a petri dish and a person differ?

The second core review, in Cancer Medicine, names the mechanism behind that difference.

Taken together, clinical follow-up, animal models and cell culture work indicate that the possible negative effect of light on tumour biology is not clinically relevant at the doses used in care ^[2]. The authors point to the contrast between studies in humans and in dishes, and to the reason for it: in a body, a tumour sits in an environment of blood vessels, immune cells and signalling molecules. In a dish it does not.

That does not make cell culture work worthless. It makes it a different question.

Is light therapy used in cancer care?

What often goes missing in this discussion: light is already used in cancer care, and not as alternative medicine.

Its main application is oral mucositis, the painful mouth ulcers caused by chemotherapy and radiotherapy. So much research exists on it that there are now umbrella reviews. The Cancer Medicine review accordingly concludes that light therapy "should be considered as part of the standard of care for specific oncology purposes" ^[2].

That is precisely why the safety question has been examined so thoroughly: it is being applied to people with cancer, so somebody had to know.

Does red light affect the chance of cancer returning?

And here is where we are sharpest, because this is where the reassurance stops.

An overview of systematic reviews from 2022 lands on the same caveat, and calls it an absence of evidence rather than a reassurance. There is too little data, and what there is is of poor quality. So it cannot be concluded that light therapy for this kind of lymphoedema is either superior or safe ^[6].

And the cell culture work itself is not uniform. Systematic reviews of light on cancer cells find genuinely mixed outcomes: sometimes reduced proliferation, sometimes increased, depending on wavelength and dose ^[3][4]. Both advise caution in cancer patients until more standardised research exists.

Has red light been tested on tumours?

One study did what the rest did not, and it deserves mention because it is the only one that combined both.

In women with lymphoedema after surgery for gynaecological cancer, far infrared radiation was tested in a randomised trial, with cell culture experiments alongside ^[7]. There was no rise in the tumour marker CA125 in either group, and no recurrence of tumour tissue or lymph node swelling was detected. The authors conclude it is oncologically safe for this group.

One study, one tumour type, one form of light. It is not an answer to the general question, but it is the kind of study that would produce the answer if it were done more often.

What we think

The answer has three parts, and quoting one of them alone gets it wrong.

The clinical data are reassuring: no trial in humans shows light therapy driving tumours, and it is given to cancer patients in hospitals. The cell culture work is not uniformly reassuring, but that work is about cells in a dish with no immune system around them. And the question that matters most to somebody who has survived cancer, whether irradiating the scar or surgical area changes the risk of recurrence, has never been studied directly.

So our advice is neither "safe" nor "dangerous". If you have or have had cancer, ask your oncologist before you shine anything on the area. Not because the evidence is alarming, but because the evidence that answers your question does not exist yet, so it has to be weighed against your own case instead of looked up, and they are the ones holding your case.