The public may have to stockpile medicines if there is a no-deal Brexit, industry leaders say.
Martin Sawer, of the Healthcare Distributors Association, told MPs industry was "very concerned" about a no-deal as it could have "catastrophic" consequences for the supply of drugs.
The government has asked firms to start stockpiling a six-week supply of drugs.
Mr Sawer said there was no need for the public to do the same "yet", but the picture could change very quickly.
The UK imports 37 million packs of medicine each month from the EU, although even more are exported out of the country.
Concern has been raised that prolonged disruption at the borders could disrupt the supply chain.
Ministers have already asked firms to start stockpiling, although that is logistically difficult for medicines that need refrigerating like insulin and vaccines, or for those with short shelf lives, which some cancer treatments have.
Appearing before the House of Commons' Health and Social Care Committee, experts said small firms in particular were struggling to stockpile drugs, as they do not have the cash flows to fund reserves of drugs.
Mr Sawer told the cross-party group of MPs: "We need politicians to understand there could be consequences. We are not suggesting anybody needs to stockpile outside of the supply chain yet.
"But come January that might be a different picture.
"We are, we believe, going to be in a difficult situation if there is not a deal by Christmas."
Mike Thompson, chief executive of the Association of the British Pharmaceutical Industry, said he did not want to alarm vulnerable patients – and drugs firms would do everything they could to get medicines to people who need them.
But he added: "Stockpiling [by industry] won't be enough."
He urged government departments to "work together" to put plans in place to alleviate some of the risks associated with a no-deal Brexit.
A Department of Health and Social Care spokesman said patients should not stockpile drugs.
He said the government was "working closely with partners" to ensure there were adequate stockpiles of medicine if there was a no-deal Brexit.
"The government is confident of reaching a deal with the EU that benefits patients, and the NHS is preparing for all situations.
"We are working closely with partners to ensure adequate stockpiles are in place for all medicines which may be affected in the event of a no-deal Brexit.
"Our number one priority is to ensure that patients have access to safe and effective medicines – and we have some of the cheapest drug prices in Europe."