There has been a "steep rise" in the number of people struggling to get hold of medication which helps control their seizures, the Epilepsy Society says.
The charity says "anxiety and stress" are putting patients at greater risk of seizures.
It is calling for the government to commission an urgent review of the medicines supply chain.
Although uncertainties around Brexit have highlighted medicine shortages, there has been a problem for years.
Last week the drug company Sanofi said there were shortages of an epilepsy drug, sodium valproate, in some areas because of supply disruption at a factory last year, and not related to Brexit.
The company added that the situation was improving.
A Department of Health and Social Care spokesperson said: "As Sanofi has made clear, these issues are unrelated to our exit from the EU and they have followed the well-established processes we have to manage the small number of supply problems that may arise at any one time."
'Not good enough'
The Epilepsy Society said rising numbers of people had been contacting the charity's helpline worried about getting hold of medication.
The drugs causing most concern are:
- Epilim (sodium valproate)
- Epanutin (phenytoin)
- Topamax (topiramate)
- Neurontin (gabapentin)
- Carbagen (carbamazepine)
- Diazepam (rectal diazepam)
Clare Pelham, chief executive of the Epilepsy Society said: "It is simply not good enough for drugs manufacturers to say 'production issues' or 'just-in-time manufacture problems' and shrug their shoulders whenever a shortage occurs.
"Surely the least that we can do – government, charities and the pharmaceutical industry – is to work together to ensure that the supply of this essential medication is reliable every day, and every month – year in and year out.
"So that when the Brexit spotlight has moved on, people with epilepsy will be in a much better place."
Epilepsy is a common serious neurological condition which affects more than half a million people in the UK.
At the same time, a leading pharmaceutical group has said there are increasing problems obtaining some medicines in England.
The number of drugs on a list of those affected by price rises and supply shortages is at the highest level since records were first compiled in 2014.
This consists of drugs where the government agrees to compensate pharmacists for higher costs. In March there were 96 medicines on the list, known as price concessions – double the number last autumn.
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