Pharmacies across the U.K. are reporting widespread shortages of aspirin, one of the most widely prescribed drugs that is used to prevent heart attacks and strokes and treat pain.

From a survey of 540 pharmacies, 86 percent had been unable to supply the medication to their patients in the past week, the National Pharmacy Association said Friday.

Pharmacies said they have been rationing supplies, prioritizing patients with the most acute heart conditions or in need of emergency prescriptions, with several saying they stopped selling the medication over the counter.

Olivier Picard, chair of the NPA, said the association is “concerned” about these reports and its implications on patients; 51 million aspirin items were prescribed in the U.K. between January and October last year.

“For those pharmacies that can get hold of supply, costs will far exceed what they will be reimbursed by the [National Health Service], yet more signs of a fundamentally broken pharmacy contract in desperate need of reform by the government,” Picard said.

Earlier this month, U.S. President Donald Trump made headlines for saying he takes a high daily dose of aspirin as it’s “good for thinning out the blood, and I don’t want thick blood pouring through my heart.”

He reportedly takes four times the recommended daily dose for cardiovascular disease prevention. On Thursday, Trump blamed the medication for his visible hand bruise at Davos.

In the U.K., pharmacists cannot offer patients substitutions for a prescribed medication, such as a different strength or formulation, without a new doctor’s prescription. The government is looking into the possibility of changing this.

“We’ve long called for pharmacists to be able to make substitutions where a medicine is not in stock and it is safe to supply an alternative,” he said. “The status quo is not only frustrating for patients, it is also dangerous.”

The government added aspirin to its export ban list on Jan. 16 amid the ongoing shortage.

In the EU, the Pharmaceutical Group of the European Union said Friday they “don’t have any signal regarding shortages of aspirin in the EU.” The European Medicines Agency, which monitors drugs in short supply, does not currently list aspirin.