“Tell me why I should have your chemotherapy when I can be healed naturally!”

His face is set, his arms defensively squared. His friend carries a pamphlet that features a suspiciously healthy woman with glamorous hair and a glowing complexion. This is the urgent appointment of the day, for whom other patients were hastily shuffled to make room.

I know I shouldn’t take the bait but, like an addict, I have the urge to say:

Go ahead then, be healed. And I will almost certainly see you again, emaciated, ruined, lamenting the fact that it’s too late.

Thankfully, the code of conduct glides in. I imagine his dread. I remember my position. And I say: “Tell me more.”

I hear about the man who uses waves, the woman who boosts immunity and the seller of pure herbs. They are the healers – 100% convincing, 100% certified by a gaggle of secret Facebook users.

He asks: “What’s the guarantee of your chemo, anyway?”

I have perfected my retort during sleepless nights.

In life there are no guarantees but you have a curable cancer. Yes, there will be side effects but we can manage them. No, I can’t guarantee a cure, but I’d recommend evidence-based treatment any day over the magnet that purportedly draws out cancer cells. And while we are there, it’s not my chemotherapy. Your taxes fund my job but I don’t profit from giving you chemo.

But how many times have I heard that if oncologists hectored a little less and listened a little more, we might win more hearts?

So I bite my tongue again, thinking of the alarmed nurse who begged me to change his mind. As I talk him through his various options from least to most intensive, I remember the patient who swapped chemotherapy for essential oils, the one who chose to “burn” the tumour out and the one who suggested I become a sales representative for a life-saving juice.

In an era where fake news abounds, why should cancer medicine be immune?

“You don’t convince me.”

“You have the facts, you get to decide,” I reply.

I used to think that these second opinions were illuminating for patients and nudged them towards change. But what I have learnt in the last few years is that cancer patients in search of alternative cures are more deeply entrenched than ever in their beliefs. Thanks to the rise of social media, the ability to filter out conflicting viewpoints and a bevy of supporters for every outrageous idea, these people arrive convinced about their theories. Arguing with them is a fool’s errand.

Despite the longest consultations, carried out by disadvantaging other needy patients, these patients are often the most dissatisfied because oncologists like me speak our truths with less conviction than the quacks who promise the world but deliver nothing except a lonely death in an unfamiliar emergency room in front of a bewildered family.

In an era where fake news abounds, why should cancer medicine be immune?

A survey commissioned by the American Society of Clinical Oncology spoke to more than 4,000 American adults, a quarter of whom were current or former cancer patients. Nearly 40% of those surveyed “somewhat” or “strongly” agreed that cancer can be cured via oxygen, diet and herbs alone. This is despite the fact that patients who solely choose alternative therapies have a greater than twofold risk of mortality, and those with early-stage cancers such as those of the breast and bowel face a four to sixfold increase in mortality compared with those who have standard therapy.

It is tempting to think that modern patients with access to multiple vetted channels of information and, equally, many sources of health warnings, will use them to their advantage. Today’s youth are thought to be especially savvy and discerning. Alas, the survey found that nearly half of those under the age of 53 thought that cancer can be cured by alternative therapies alone. Even among people directly affected by cancer, a quarter believed in alternative therapy over standard treatments. If anyone was relying on family to help them see the light, more than a third of caregivers for cancer patients shared the misguided belief in alternative cures.

Enzymes, waves and magnets do not cure cancer, and they cost the patient every step of the way. Small bottles of unknown and frequently adulterated or plainly toxic substances cost hundreds of dollars, not to mention every consultation that pretends to read the eyes and sense the energy to cure cancer, even as the patient worsens. How do I know? Because dying patients relate these stories in a last attempt to prevent their fellow patients being duped.

It has been received wisdom that oncologists can see off quackery through good communication but I’m afraid that isn’t so.

Oncologists have been properly entangled in a web of fake news. Their authority has been undermined and their expertise ridiculed by a determined, global and hard-to-track battalion of quacks and their acolytes. Greater vigilance, stronger regulation and improved health literacy might help, but the pull of alternative cures is strong.

Make no mistake. With so much misinformation fuelling the use of increasingly bizarre alternative therapies, patients will be ultimately robbed and disappointed, and their doctors will be relegated to the sidelines. To paraphrase an old joke, oncologists will no longer be giving chemotherapy until the grave, but the quacks will be laughing all the way to the bank.

• Ranjana Srivastava is is an oncologist and Guardian Australia columnist