We Were Wrong About Aspirin (New Evidence)

The speaker opens and closes the video with forceful warnings about secondary prevention. He distinguishes it from primary prevention—the focus of the entire episode. He states that the evidence for aspirin in secondary prevention is robust and that discontinuing it without medical guidance can be fatal. The ASPREE and Cochrane data only address people without a history of cardiovascular events. He explicitly says: 'None of what I've said today in this video changes that.' This protocol is not new but is emphasized to prevent harm from misinterpretation.

The speaker walks through the ASPREE data in detail, concluding that the case for aspirin in primary cancer prevention has collapsed for older adults. He highlights that the harm signal—15% more cancer deaths—disappears when aspirin is stopped, so removal can mitigate risk. He also stresses the well-established 59% increased risk of serious extracranial bleeding, rated as high-certainty by Cochrane. This, combined with the cancer mortality signal, makes the benefit–risk ratio unacceptable. For clinicians like the speaker, aspirin is not used for this indication.

The speaker contrasts ASPREE with CaPP2 to illustrate that aspirin can work for cancer prevention under very specific conditions: a high genetic risk (Lynch syndrome confers up to 60% lifetime colorectal cancer risk), a younger starting age (median 45 vs 74 in ASPREE), a much higher dose (600 mg vs 100 mg), and longer follow-up. The results were ‘staggering’—a 35% reduction in colorectal cancer. This validates Peter Rothwell’s original findings in a targeted population. The speaker warns against self-treatment; this protocol requires medical guidance.

The speaker acknowledges that the Rothwell meta-analyses, which showed benefit emerging after 5–7.5 years and beyond, might still apply to a younger cohort willing to stay on aspirin for decades. However, because ASPREE enrolled people ≥70 and the Cochrane review focused on older populations, the data for a 40–50-year-old with a strong family history is indirect. The speaker says, 'I'm sorry, there isn't a clear answer.' He advises an individualized discussion with a doctor, balancing family cancer history against personal bleeding risk. This contrasts sharply with the clear 'stop' message for older adults.

ASPREE enrolled 19,114 people over 70 (or over 65 for black and Hispanic individuals) with no cardiovascular disease, dementia, or disability—the healthiest cohort of older adults possible. Half received 100 mg aspirin daily, half placebo, with 2,000 GP practices tracking outcomes. After 8.6 years, 3,448 new cancers and 1,173 cancer deaths were recorded. Cancer incidence was identical (HR for colorectal cancer 1.01). Cancer deaths were 15% higher in the aspirin group. Intriguingly, when researchers examined the 4.5-year period after participants stopped aspirin, the increased cancer mortality had disappeared, indicating the harm was reversible and dependent on ongoing exposure. The speaker describes opening the spreadsheet expecting a positive or null result and finding the opposite of what Rothwell predicted. This result, combined with the well-known 59% increased risk of serious extracranial bleeding (high-certainty Cochrane evidence), has collapsed the justification for aspirin in primary cancer prevention among older adults.

The review pulled together randomized controlled trials that tracked cancer outcomes in adults without prior cardiovascular disease. In the 5- to 10-year window—the relevant timeframe for most older adults who started aspirin in their 60s or 70s—colorectal cancer mortality was 77% higher in the aspirin group, though the certainty of that finding was rated low. The authors did detect a possible reduction in colorectal cancer at 15 years or longer of continuous use, but they rated that evidence as very low certainty, the weakest grade. This means that any potential protection after prolonged use remains speculative and unconfirmed. The speaker notes that these findings, together with ASPREE, have already led the US Preventive Services Task Force to quietly withdraw its colorectal cancer recommendation in 2022, and no major guideline body currently endorses aspirin for cancer prevention in the general population.

The speaker explains that cancer cells traveling to seed metastases surround themselves with platelets. Those platelets release thromboxane A2, which puts the brakes on T cells—the immune cells that would otherwise kill the cancer cells. Aspirin at low doses shuts down platelet production of thromboxane A2, so in theory it should release the brakes and boost anti-cancer immunity. The Cambridge mouse experiments confirmed exactly that. Yet in the ASPREE trial of healthy 70-year-olds, aspirin led to more cancer deaths, not fewer. The ASPREE team proposed three hypotheses: (1) Immunosenescence—by age 70, T cells are less responsive, so removing the thromboxane A2 brake doesn't help; (2) Aspirin's anti-inflammatory effect might mask early cancer symptoms like aches and fever, delaying diagnosis (though researchers judged this unlikely); (3) Chronically suppressing inflammation over years might quiet the body’s normal surveillance that isolates developing tumors, giving them room to grow. All three may contribute, but the precise reason remains unknown. The speaker emphasizes that the drug therefore performs differently in an aged immune system than in young mice or middle-aged trial participants.

The speaker notes that the task force withdrew the colorectal cancer recommendation after the initial ASPREE results showed not just a lack of benefit but a hint of increased cancer mortality. By 2022, they had dropped it. As of the video's recording, no major guideline body recommends aspirin specifically for cancer prevention in average-risk individuals. This marks a complete reversal of the narrative that had guided millions of pharmacy visits.

In 2020, the CaPP2 trial published 10-year follow-up results in The Lancet. Participants were Lynch syndrome carriers, median age 45, taking 600 mg aspirin daily (six times the typical 100 mg low dose). The result was a staggering 35% reduction in colorectal cancer. This trial used a dose and age profile much closer to Rothwell’s original data. The speaker frames this as proof that Peter Rothwell was not wrong; he found a real effect that was then misapplied to the wrong group. The lesson: aspirin can prevent cancer in the right person, at the right dose, started young enough, and continued long enough. For healthy 74-year-olds on 100 mg for five years, it doesn't work and may increase mortality.