Medical suppliers have been warned of border disruption for up to six months in the event of a "no-deal" Brexit.
In a series of letters to industry, Health Secretary Matt Hancock set out "revised" estimates over the impact on cross-Channel ports of the UK leaving the EU without an exit agreement.
He wrote: "The revised cross-government planning assumptions show that there will be significantly reduced access across the short straits, for up to six months."
Mr Hancock stressed "this is very much a worst-case scenario" but "as a responsible government, we have a duty to plan for all scenarios".
He added, if a no-deal Brexit were to occur, the government would push EU member states "hard" to introduce "pragmatic arrangements to ensure the continued full flow of goods".
In August, Mr Hancock urged pharmaceutical companies to stockpile an extra six weeks' worth of drugs in case of a no-deal Brexit.
The health secretary has now suggested such preparations may not be enough due to the government's "changed" assessment.
One of the letters added: "Whilst the six-week medicines stockpiling activities remain a critical part of our UK-wide contingency plan, it is clear that in light of the changed border assumptions described above this will now need to be supplemented with additional actions."
However, Mr Hancock urged local health and social care providers not to stockpile themselves, warning it is "unnecessary and could cause shortages in other areas, which would put patient care at risk".
The letters also confirm the government is planning to fly in medicines with a short shelf life, such as medical radioisotopes used to treat cancer.
Medicines and medical products will also be "prioritised" on "alternative routes" for lorries, Mr Hancock added.
A government source stressed the letters are not connected to next Tuesday's vote on Theresa May's Brexit deal and would have been published anyway.
Brexiteer critics of the prime minister's agreement have recently claimed a new "Project Fear" is being pursued in order to convince MPs to vote for her deal.
Industry figures called for "immediate action" from government following Mr Hancock's "stark" warning.
Mike Thompson, chief executive of the Association of the British Pharmaceutical Industry, said: "Pharmaceutical companies continue to do everything in their power to make sure that patients get access to medicines whatever the Brexit scenario.
"This includes duplicating processes, changing supply routes and stockpiling medicines in line with the government's guidance.
"However, we have been clear that there are things which are out of our control.
"Today's update on potential border delays for six months in a 'no-deal' scenario is stark. Stockpiling more medicines is not the solution to this problem.
"While we welcome the secretary of state's intention to prioritise the flow of medicines and vaccines, we need the detail.
"With just 16 weeks until the UK leaves the EU, we need the government to take immediate action to open up alternative supply routes between the UK and Europe and tell companies so that they can make plans."
More from Brexit
In a report setting out Kent County Council's preparations for a no-deal Brexit, it was revealed the government has increased its no-deal planning scenario from three to six months of disruption.
"The rationale for this has not been provided by government," the document, seen by Sky News, added.