A global shortage of remdesivir, one of the key Covid-19 drugs given to Donald Trump since he tested positive for the virus, is leading to rationing in the UK and pressure on the manufacturer to allow other companies to supply it.

Remdesivir, made by the US company Gilead, is one of only two drugs proven to work against Covid-19. It has been shown to help patients recover faster from the disease and shorten the length of hospital treatment but is not a cure.

Unlike the steroid dexamethasone, which saves lives and is widely available and cheap because it is an old drug, remdesivir is under patent, which means no other company can make it without Gilead’s permission.

Since trial results showed it had an effect in Covid-19, Gilead has been unable to keep up with demand.

The US in June bought up virtually all stocks for a three month period, but even US hospitals say they cannot get enough to treat their patients. In Europe, where the European commission buys on behalf of member states, the Czech Republic and the Netherlands as well as the UK are short of supplies.

Doctors in the UK have been instructed to treat only those patients who can most benefit from the drug, in order to conserve its limited stocks, which were bought earlier in the year with trials in mind.

“We are aware of a rise in the use of remdesivir in line with an increase in Covid-19 hospital cases,” said a Department of Health and Social Care spokesperson.

“While there remain plenty of remdesivir supplies, we have asked the NHS to temporarily prioritise patients to ensure those most likely to benefit can access it.”

The department issued a “supply disruption note” to all doctors on 29 September. It asked them to do a stock-take in their hospitals to ensure they knew how much of the drug they had left. Some stocks would be held centrally, it said, for allocation “to areas of greatest need”.

Doctors should prescribe remdesivir only to patients with Covid-related pneumonia who are in need of oxygen to breathe and should limit any course of treatment to five days, they were told.

Dr Andrew Hill, senior visiting research fellow at Liverpool University, said the crisis was predicted in June, when the US secured all production of the drug until October. He had heard, he said, that some UK hospitals were down to their last few doses of the drug. Others were figuring out how to ration it, for instance by giving fewer than five doses if a patient seemed to be doing well.

The criteria for using remdesivir given in the supply disruption note meant it could not be used in the NHS before people had developed pneumonia, in spite of evidence that giving the antiviral as soon as possible is better.

“If Donald Trump was in an NHS hospital, he wouldn’t be eligible,” Hill said. In the US, the Food and Drug Administration had written to Gilead broadening the criteria for use, so that people can be given it whether they are on oxygen or not, which will also increase demand.

But while wealthy countries are running out, there is no shortage in many low-income countries. Gilead, the patent holder, has granted licences to nine generic manufacturing companies in Egypt, India and Pakistan. But they are not allowed by the company to sell to high income countries.

Hill says one solution would be for a country such as the UK to issue a compulsory licence, which would bypass Gilead’s patent and enable it to buy from one of the generic companies. World Trade Organization rules allow for this in situations of need, but it is very rarely used.

Another would be to launch a trial of the generic version, as the NHS did when it faced a huge potential bill for another Gilead drug called Truvada, used in PreP – pre-exposure prophylaxis – to protect people at risk of HIV infection. The NHS Impact Trial enrolled 26,000 people on a generic version of the drug.

“Gilead can’t stop the UK doing it,” said Hill. “They could try to sue them, but the ball is in Gilead’s court to supply enough for the whole of Europe.”

Gilead said it had been ramping up production of remdesivir – brand name Veklury – since January “in order to meet the potential global demand due to Covid-19 by rapidly increasing our manufacturing capacity around the world – including our own manufacturing sites and external organisations in partnership with industry peers.

“We anticipate from October onwards we will have sufficient supply to meet global demand. We are working with the European commission to provide equitable access to remdesivir for patients who need them most across several European countries, including the UK, and also with the Department of Health to understand the current demand and to address the needs as quickly as possible.”