1228 GMT April 22, 2019
This is leaving UK patients complaining of delays in getting hold of drugs such as painkillers, anti-depressants and blood pressure medication.
The BBC has found there has been a big rise in the number of drugs on the ‘shortage of supply’ list for England.
There are 80 medicines in such short supply that the Department of Health has agreed to pay a premium for them.
This is up from 45 in October.
There are a number of reasons why this has happened, but there are now concerns that uncertainty over Brexit will only make the situation worse.
The Royal Pharmaceutical Society said there was ‘a massive shortage and price spikes’.
What does this mean for patients?
Most people should be able to get their prescriptions filled as normal.
But if they need one of the drugs that is running short, they might not be so lucky.
Some pharmacists are sending patients back to their GPs to ask for a different medicine or dosage.
Others are giving as much of a drug as they can spare and sending people away with IOU notes for the remainder.
The best advice is to make sure you get prescriptions to your pharmacist in good time.
It is almost always possible to come up with an alternative.
However, that can be more difficult with conditions like epilepsy, where patients need to be on specific drugs.
What is the scale of the problem?
It is hard to obtain a definitive tally of which medicines are running short.
But the industry in England uses a list from the Pharmaceutical Services Negotiating Committee (PSNC).
It shows which drugs are in such short supply and for which ones the NHS has agreed to temporarily pay a higher price.
The PSNC publishes monthly lists of these so-called concession prices.
The BBC has analyzed this data and found that the number of medications on the list has grown six-fold in three years.
During this time period the peak was in November 2017, but there has been a recent surge and figures for December show it is approaching that level again.
What drugs are affected?
This is about prescriptions for generic medicines, rather than specific brand names.
For example, Nurofen is a common painkiller, but you can buy the same generic drug, ibuprofen.
Ash Soni, president of the Royal Pharmaceutical Society, said he has never seen so many common drugs affected by shortages.
He explained: "The items are out of stock and unavailable. Patients are having to wait.
"We're having to send some patients back to the GP to get a different prescription, because we just can't fill them."
On December 2018's concession list, 28 drugs, or about a third, were among the 500 most commonly prescribed.
For example, furosemide is used to treat high blood pressure and other cardiovascular problems.
It comes in various dosages, but one type, 40mg tablets, is the 23rd most commonly prescribed drug in England.
Other drugs on the list include fluoxetine, which treats depression, and naproxen, which is an anti-inflammatory.
Soni said naproxen went ‘completely out of stock’ recently.
"I didn't have any on my shelf last week. I've got patients who are waiting for it."
He did manage to track some down eventually, but it cost £6.49 a box. That is £2 more than the NHS last agreed to pay for it.
He said: "I've ordered 20 boxes today and that will last me about two or three days.
"We're dispensing at a loss. We're paying for patients to get their meds on the behalf of the NHS."
Warwick Smith, director general of the British Generic Manufacturers Association, said stock levels can fluctuate.
He prefers to call it a ‘tightening of supply’ rather than a shortage.
"It's normal for levels of availability to increase and decrease, which impacts prices," he adds.
The government stresses that two million prescription items are dispensed in England every day, and the vast majority of medicines are not in short supply.