Health misinformation on Facebook was viewed 3.8 billion times in the past year, peaking during the Covid-19 crisis, a report suggests.
Activist group Avaaz, which conducted the research, said Facebook posed a "major threat" to public health.
Doctors added false claims about vaccines on the social network could limit the numbers prepared to have a Covid jab if one became available.
Facebook said the findings did "not reflect the steps we've taken"
In a statement the firm said: "We share Avaaz's goal of limiting misinformation. Thanks to our global network of fact-checkers, from April to June, we applied warning labels to 98 million pieces of Covid-19 misinformation and removed seven million pieces of content that could lead to imminent harm.
"We've directed over two billion people to resources from health authorities and when someone tries to share a link about Covid-19, we show them a pop-up to connect them with credible health information."
Despite its efforts, Avaaz's report suggests only 16% of the health misinformation it identified on Facebook carried a warning label.
The top 10 websites identified by researchers as spreading health misinformation had almost four times as many views on Facebook as information from official sites, such as the World Health Organization, according to the report.
A significant amount of health misinformation was shared from public pages.
And 42 of those pages were followed by more than 28 million people.
The type of content highlighted includes:
The research looked at accounts from the US, UK, France, Germany and Italy.
Avaaz campaign director Fadi Quran said: "Facebook's algorithm is a major threat to public health.
"Mark Zuckerberg promised to provide reliable information during the pandemic.
"But his algorithm is sabotaging those efforts by driving many of Facebook's 2.7 billion users to health-misinformation-spreading networks.
"This info-demic will make the pandemic worse unless Facebook detoxifies its algorithm and provides corrections to everyone exposed to these viral lies."
Avaaz called on the social network to provide everyone who had viewed health misinformation on its platform with independently fact-checked corrections.
It also said it should downgrade misinformation posts in news feeds, in an attempt to lessen their reach.
Prof Frank Ulrich Montgomery, who chairs the World Medical Association, said: "This pandemic should be a powerful reminder of how successful vaccines have been.
"But instead anti-vaxxers are using Facebook to spread toxic lies and conspiracy theories."
European Union of Medical Specialists secretary general Dr Joao Miguel Grenho said: "Mark Zuckerberg must take immediate action to stand with us to stop this info-demic.
"Otherwise the number of people poisoned against taking a vaccine will be too high for us to beat this pandemic."
Facebook insists it is combating dangerous health myths.
And if you put the word "vaccines" in its search box, you do get pointed towards reliable information.
But it is also easy to find thousands of groups and individuals spreading misinformation with no apparent intervention from Facebook.
A case in point is a page called Kate Shemirani - who calls herself Natural Nurse in A Toxic World and stresses her qualifications.
In fact, Ms Shemirani's nursing registration was suspended by the Nursing and Midwifery Council in July, after complaints she was spreading false information about Covid-19 and about vaccines.
A glance at what she posts on Facebook and Twitter reveals relentless attacks on Bill Gates, the wearing of masks and NHS staff, whom she calls criminals and liars for perpetuating the "hoax" that is the coronavirus.
But much of her content is about vaccines - this week she claimed the polio vaccine caused polio and had harmed and killed thousands.
We forwarded this post to both Facebook and Twitter to ask if it broke their rules.
While Twitter would not give a public statement, it indicated its rules covered Covid-19 only, so did not affect this post.
Facebook said that while it doesn't prevent people from saying things that are factually incorrect, it has acted to make this and similar posts less prominent in people's news feeds.