Pharmacists have been having stock problems for months
Pharmacists are warning that patients are suffering because of a shortage of some medicines.
An attractive export market and quotas set by drug manufacturers are being blamed for ongoing stock problems with more than 50 branded medicines.
A third of pharmacies polled by Chemist and Druggist magazine said shortages were now directly affecting patients, with one patient ending up in hospital.
The government said it was "monitoring the situation closely".
More than 150 pharmacies responded to the survey, almost 90% of whom said they were "very concerned" about problems they were having getting hold of some medicines.
And 32% said they had already seen patients suffer because of stock shortages.
Some reported patients being extremely distressed when they were not able to obtain their prescribed drugs.
Others had seen a patient's condition deteriorate with one even being admitted to hospital.
The vast majority said they had had to ask a GP to rewrite a prescription for a different drug when they could not get hold of stock.
One pharmacist reported a patient suffering panic attacks waiting for an antidepressant called Cipralex and another had been extremely distressed while waiting for Femara, which is a drug used to treat breast cancer.
Another said a patient's blood pressure had gone up while waiting for their medicine and that a shortage of Plavix, a clot-busting drug, led to a patient being admitted to hospital.
Jeremy Holmes, chief executive of the Royal Pharmaceutical Society of Great Britain, said it was concerned about the stock problems.
"This problem has been exacerbated recently by parallel exporting, where a product is bought in the UK in order to sell onto another country at a higher price."
He added: "The current supply chain relies on medicines being available within 24 to 48 hours.
"When this doesn't happen it causes stress and anxiety to those affected - and it is the patients who suffer most."
A spokesman for the Department of Health said it was vitally important that people get the medicines they need and were aware of the issue.
"We are working with pharmacists, pharmaceutical companies and wholesalers to better understand these concerns, and will continue to monitor the situation closely in order to address these."
Conservative and Liberal Democrat MPs condemned the situation as unacceptable. Howard Stoate MP, chair of the All Party Pharmacy Group, said the group was "extremely concerned" that patients were unable to access medicines.
Roger Odd, a trustee of the Patients' Association, said: "There are 50 or more medicines that have been out of stock - it's unbelievable.
"It can't be right for patients to be suffering like this."
David Fisher, of the Association of the British Pharmaceutical Industry, said patients were being put at risk because medicines intended for UK patients are being diverted for export.
"Industry very much shares patient concerns and has been working with Department of Health and pharmacy bodies on solutions."