Patients struggle to obtain drugs for life-threatening conditions in 80% of NHS trusts in England and Wales, a survey suggests.
Delays are reported in four out of five trusts because medicines intended for NHS patients are exported to other EU countries, causing shortages.
Quotas have been introduced to control the amount of drugs set aside for the NHS but this has been found to exacerbate the problem.
Labour MP Huw Irranca-Davies is calling for action from both the Government and industry, after more than 60 health authorities responded to his freedom of information request about drug shortages.
The member for Ogmore in Wales wrote on Twitter: "Not good enough: over 60 responses from health authorities show gov't & industry need to act on medicine shortages."
The Department of Health said it is considering an investigation into the scale of the problem.
A spokesman said: "It is for manufacturers to ensure that any quotas set are flexible enough to cope with reasonable fluctuations in demand. Where quotas are used, companies setting them should also make sure that they are set fairly.
"Manufacturers should have their own contingency arrangements in place for pharmacies to be able to obtain medicines directly if they have problems getting hold of them.
"The Government does maintain a buffer stock of certain essential medicines that can be released in emergencies. We will take any action necessary in the event of disruption to supply and distribution of medicines that causes serious risk to patients."
In April health minister Simon Burns told Parliament that the department is considering a one-off survey of shortages of medicines at local pharmacies.
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