Twitter says it will remove content that promotes unverified claims and misinformation about coronavirus.
It said tweets that could put people at a higher risk of contracting Covid-19 would be banned under the revised safety rules.
That includes unverified claims that specific groups are more susceptible to coronavirus.
It also covers denial of expert guidance, and unofficial advice about ways to diagnose or treat the virus.
“We will enforce this in close co-ordination with trusted partners, including public health authorities and governments, and continue to use and consult with information from those sources when reviewing content,” Twitter said in a post on its website.
Harmful content will be triaged, it added, prioritising removing tweets that “present the biggest risk of harm”.
However, the BBC's technology correspondent, Rory Cellan-Jones, said there was no specific option to report tweets of this nature.
“Given the sheer volume of disinformation from everyone from anti-5G campaigners to homeopaths pushing fake remedies, Twitter has set itself what looks like an impossible task,” he said.
“It may also find itself accused of Western or liberal bias, as has happened with previous attempts to crack down on other kinds of disinformation.
“And it is not clear how it expects the Twitter community to help – none of the categories for reporting a tweet specify misinformation.”
It follows an announcement from Facebook, Google, Microsoft and Reddit earlier this week, in which they committed to work with governments to fight misinformation and to help connect those in self-isolation.
“We are working closely together on Covid-19 response efforts," the joint-statement said.
“We’re helping millions of people stay connected while also jointly combating fraud and misinformation about the virus, elevating authoritative content on our platforms and sharing critical updates in co-ordination with government healthcare agencies around the world.”
Social media sites and search engines are already placing official guidance from the NHS and World Health Organization at the top of search results.