Some offer stem cell treatments for brain disorders for tens of thousands of pounds. Others sell cures for multiple sclerosis and cancers.
But the evidence behind the remedies is often unreliable, experts say, and patients are increasingly being exploited.
Experts and patient groups want to see tighter regulation to reduce unfounded claims.
Dr Kieran Breen, director of research at the Parkinson's Disease Society, said: "It can be tempting to believe personal stories of miracle cures, but only by using tried and tested methods can we move forward and provide people with Parkinson's with the best available advice and treatments."
'Looking for hope'
Lib Dem MP Phil Willis, chairman of the Innovation, Universities, Science and Skills Select Committee, said the "cruellest deception" for a patient with chronic illness was the promise of a cure based on "empty hope not evidence".
Ian Douglas, who has multiple sclerosis, agreed, saying patients with incurable conditions found hope in short supply and looked for anything that might help.
"This makes them particularly vulnerable to treatments that are supported only by anecdote and rumour," he said.
A new guide, called I've got nothing to lose by trying it, has been published by Sense About Science with the Multiple Sclerosis Society, Motor Neurone Disease Association, Alzheimer's Society and Parkinson's Disease Society.
It aims to help patients tell the difference between beneficial remedies and bogus treatments.
Hazel Thornton, a patient advocate who acts as a liaison between patients and healthcare providers, said the guide would help people "bring a critical eye and a questioning mind to what they read and hear".
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.