For purveyors of snake oil, January is an especially lucrative month. Fresh from our festive excesses, our bodily insecurities are readily exploited by those looking to make a quick buck from a worthless ware. This month has already seen the unedifying sight of a Kardashian pushing a “flat-tummy” shake to her 27.7 million Twitter followers, while Netflix has abandoned any pretension of quality-control by allowing the actor Gwyneth Paltrow her own show, effectively a glorified promotional vehicle for Paltrow’s lifestyle brand Goop.

For those so inclined, Paltrow sells all manner of pseudoscientific regimes, from “jade eggs” (essentially rocks that Paltrow recommends women carry in their vagina, despite gynaecologists imploring people not to do this) to “healing stickers” to a $75 candle that ostensibly smells of Paltrow’s vagina. Sadly, neither repeated admonishment by scientists and doctors nor a lawsuit against Goop for medically unsupported claims have done anything to impede their popularity, and the candles have already sold out, as did the jade eggs before them.

Across social media, influencers perpetuate wellness trends and dubious diets, frequently promoting completely useless or even dangerous advice. The perennial market for “detox” and “cleanse” diets and products is a recurrent theme. As has been written frequently here, these are completely useless from a scientific standpoint, given our liver and kidneys filter toxins quite admirably. Despite this, detox products and their offspring diets top $5bn in sales annually, driven to a large extent by celebrity and influencer endorsement.

If celebrity itself is a nebulous term, influencer is an even more amorphous and vague dystopian concept; one fitting and ominous definition describes the role as a “third party who significantly shapes the customer’s purchasing decision but may never be accountable for it”.

It’s tempting to laugh and utter something about fools and their money, but it is undeniable that celebrity influence has a tangible effect on public perception. Jade Goody’s untimely death from cervical cancer in 2009 is a case in point. During her ordeal, appointments for cervical smears surged by up to 70% above projected figures, resulting in an estimated extra 500,000 screenings in the UK. Dubbed the “Goody effect”, the rise was particularly pronounced in those from lower socioeconomic groups, whom public health campaigns frequently miss.

The reason for the surge was probably down to the availability heuristic, the phenomenon by which immediate and easily accessed information is afforded a greater weight. Consequently, we tend to skew our judgments toward more recent information, biasing our opinions towards the latest news or most easily accessed example. But conversely, as examples become less vivid and recent they dissipate from our collective consciousness. By 2017, once public memory of Goody’s plight had faded, cervical screening in the UK fell to a 19-year low.

Crucially, celebrity coverage appears to be what affects screening statistics, not any inherent benefit of the test itself. And unfortunately, this increased focus is not always beneficial; following Kylie Minogue’s breast cancer diagnosis in 2005, requests for mammograms in Australia more than doubled. Yet this was in many respects detrimental, as the vast majority of these requests came from young women, for whom screening was likely to yield false positives and needless invasive procedures.

Nor did the increased discussion of breast cancer correlate with improved understanding; as with most cancers, age is the greatest risk factor for breast malignancies, and the overwhelming majority occur in older patients. As Lesley Walker of Cancer Research UK cautioned, the net effect of celebrity-skewed coverage can “set up a chain of panic among young women, while misleading older women to think that ageing is not a relevant factor in breast cancer”.

But while endorsing relatively inert quackery is one thing, it is an order of magnitude worse when a celebrity uses their platform to push debunked nonsense. This is perhaps most prevalent on vaccination, where it is not uncommon for celebrities to push discredited and dangerous narratives on social media and on air. A non-exhaustive list would include Jenny McCarthy, Jim Carrey, Rob Schneider, Alicia Silverstone, Bill Maher and Jessica Biel.

The most disappointing of these figures might be Senator Robert F Kennedy Jr, who despite persistent criticism from the medical-scientific community and even his own family, has for years scare-mongered about vaccination. This goes far beyond the peddling of discredited myths – a recent study found a large number of anti-vaccine adverts on Facebook were funded by a group led by Kennedy. As I’ve written recently for the BMJ, such misinformation is behind the dark renaissance of diseases that had once virtually been conquered, leading the WHO to declare vaccine hesitancy among the top 10 threats to public health in 2019.

This is not to say that all celebrity influence is a negative – there are numerous examples of well-known figures using their profile for good. Roald Dahl, who lost a daughter to measles, penned a heartbreaking missive encouraging parents to vaccinate. Elvis Presley being vaccinated against polio at a press conference in 1956 encouraged uptake of the vaccine in the then under-represented teenage cohort, and actors such as Alan Alda have done incredible work in advancing public understanding of science.

The reality is that for better or worse, we do not exist in a vacuum, and our perceptions are more influenced by external coverage than we might realise. But in an era where repackaged snake oil can be easily and immediately spread by anyone with a platform, we must be more mindful than ever to question dubious claims. We should make healthy scepticism our default position; our continued wellbeing depends on it.