Sunscreen does prevent skin cancer. One Australian trial carries most of the evidence

Sunscreen reduces the risk of squamous cell carcinoma, and there is randomised evidence that it reduces the risk of invasive melanoma too. Most of that evidence comes from a single community trial of 1,621 adults in Australia, which is a narrower foundation than the confidence of the public messaging implies — and the number printed on the bottle does not mean what most people think it means.

What the trials found

A review in CMAJ assembled the randomised evidence [s1]. The central study is a community-based trial that randomised 1,621 adult Australians for four and a half years to either daily sunscreen at the recommended application or sunscreen at their own discretion, then followed them for more than a decade [s1]. Participants assigned to daily use had a 40% lower incidence of squamous cell carcinoma, a rate ratio of 0.61 (95% confidence interval 0.46 to 0.81) [s1].

Basal cell carcinoma, the most common skin cancer of all, was not significantly reduced — the review's suggested explanation is the long, protracted course by which basal cell carcinomas develop [s1]. Almost fifteen years after the trial ended, participants who had used sunscreen daily throughout it showed a significantly reduced risk of invasive melanoma, a hazard ratio of 0.27, though the review is explicit that very few invasive melanomas were observed given how long melanoma takes to appear [s1].

A second large Australian trial, running over a single summer season, found a reduced rate of actinic keratoses — the precursor lesion to squamous cell carcinoma — in participants randomised to regular sunscreen against a non-active base cream, a rate ratio of 0.62 [s1]. In organ transplant recipients, a group at high risk of dying from skin cancer, a prospective single-centre study of 120 matched patients found that SPF 50 sunscreen over 24 months reduced actinic keratoses and squamous cell carcinomas, and to a lesser extent basal cell carcinomas [s1].

A note on the source: both confidence intervals above are printed in the review with what appears to be a typesetting error, the upper bounds rendered as negative numbers. A rate ratio cannot be negative. The point estimates are unaffected.

Where the evidence is genuinely contested

Recent meta-analyses have not reproduced these trial results, finding no significant effectiveness of sunscreen for preventing either melanoma or non-melanoma skin cancer [s1]. The review's counter-argument is methodological: those meta-analyses pooled retrospective studies with inconsistent methods, and one of them included studies testing sunscreens that filtered only UVB rather than the broad-spectrum products now sold [s1]. Its conclusion is that the highest-quality evidence available supports prevention [s1].

A 2026 narrative review reaches a more graded version of the same position: daily sunscreen use significantly reduces actinic keratoses and cutaneous squamous cell carcinoma in high-risk individuals, while the evidence for basal cell carcinoma and melanoma prevention "remains heterogeneous" [s3]. Both readings are defensible from the same literature. The honest summary is that the squamous-cell finding is solid, the melanoma finding rests on a small number of events in one trial, and the basal-cell finding is absent.

One gap is worth stating plainly. There have been no studies assessing whether regular sunscreen use reduces skin cancer risk among people who are not White, even though guidance recommends sunscreen for people of all skin types [s1].

What SPF actually measures

SPF is a sunburn measure, not a time measure. Every sunscreen is tested for how much ultraviolet exposure it takes to cause sunburn while using the product compared with how much it takes without it, and the resulting number is printed on the label [s2]. Because sunburn is predominantly a UVB phenomenon, SPF primarily indicates a product's UVB protection [s2].

The FDA is direct about the most common misreading: some people believe that if they normally burn in one hour, an SPF 15 product buys them fifteen hours. That is not what the number means, because SPF relates to the amount of solar exposure, not its duration [s2]. Solar intensity varies with time of day and with latitude, so the same clock time delivers different doses in different places [s2].

Protection against UVA is a separate claim, carried by the words "broad spectrum" rather than by the SPF figure [s2]. Products that are neither broad spectrum nor at least SPF 15 must carry a warning stating that they have been shown only to help prevent sunburn, not skin cancer or early skin ageing [s2].

The application problem

Observational studies consistently find that people apply between 20% and 50% of the quantity used in SPF testing [s1]. A product labelled SPF 50 applied at real-world thickness may deliver around SPF 25 [s1]. The FDA's own figure is that an average-sized adult needs at least one ounce of sunscreen to cover the body from head to toe [s2]. Split-face studies have found SPF 100 superior to SPF 50 for preventing sunburn under actual use conditions, at the beach and at altitude on skis [s1] — a finding that reads as evidence about underapplication as much as about the filters themselves.

Reapplication is one place the sources disagree, and the disagreement is worth seeing. The FDA advises reapplying at least every two hours and more often when swimming or sweating [s2]. The CMAJ review cites recent experimental work finding that sunscreen remains on the skin at its labelled SPF for as long as eight hours after a single application, and suggests the historical two-to-three-hour advice need not be followed even during physical activity — while noting that reapplication is still called for when the product is likely to have been physically removed by sweat, water, sand or clothing [s1]. Neither source is describing a different fact; they are weighting the same one differently.

What sunscreen is not

Sunscreen is one component of photoprotection, not the whole of it. The review's list of the rest is unglamorous: avoiding the midday sun, shade, wide-brimmed hats, wrap-around sunglasses, and tightly woven, darker clothing, which loses protective value when wet or stretched [s1]. Products sold on antioxidant claims are treated sceptically in the same review, which notes that antioxidants cannot yet be stabilised inside sunscreen formulations and that studies have found sunscreens claiming antioxidant activity to have little to none [s1].

This article is informational and is not medical advice.

Sources

• [s1] The efficacy and safety of sunscreen use for the prevention of skin cancer — CMAJ, 13 December 2020. https://doi.org/10.1503/cmaj.201085
• [s2] Sunscreen: How to Help Protect Your Skin from the Sun — US Food and Drug Administration, content current as of 9 June 2026. https://www.fda.gov/drugs/understanding-over-counter-medicines/sunscreen-how-help-protect-your-skin-sun
• [s3] Photoprotection for Skin Cancer: What's New — Cancers, 15 February 2026. https://doi.org/10.3390/cancers18040634