In recent months, large drugmakers including Allergan, Gilead and Novartis have collectively spent billions of dollars acquiring or licensing medicines designed to treat a liver disease that few people have heard of — non-alcoholic steatohepatitis, or Nash.

In the most serious cases, the illness causes fatal cirrhosis, while also increasing a person’s chances of developing liver cancer or heart disease. The US Centers for Disease Control believes there are roughly 20,000 fatalities each year from chronic liver disease or cirrhosis that are not related to alcoholism.

Drugmakers are betting that the number of people with Nash — which is more common in overweight people — will rise dramatically in the coming years because of the worldwide obesity epidemic. Some analysts are predicting the global market for Nash medicines will be worth as much as $35bn a year at its peak.

There are no approved drugs to treat the condition but pharmaceutical groups are studying more than 25 experimental compounds in humans, with four medicines either being studied in phase III clinical trials, or about to enter this final stage of testing.

Of the large drugmakers, Allergan is furthest ahead, having spent $1.7bn to acquire Tobira, a San Francisco biotech group, in November last year. Tobira’s main medicine, Cenicriviroc, will enter phase three trials later this year with results expected as early as 2019.

Allergan recently acquired another smaller Nash-focused drugmaker, Arkana, for $50m.

“Nash is now the leading cause of liver cirrhosis and cancer, having taken over from hepatitis C,” says Dr David Nicholson, Allergan’s chief research and development officer. “And at the moment, there’s nothing out there to treat it.”

Gilead has entered the field too, recently acquiring Nimbus for up to $1.2bn, while also investing heavily in its internal research efforts. The West Coast-based biotech company has already had huge success with its treatments for hepatitis C, another potentially fatal liver disease.

Several other well-known pharmaceutical groups, including Novo Nordisk and Shire are working on Nash drugs at an earlier stage of development.

However, the two companies that are most advanced do not come from the ranks of big pharma: Intercept Therapeutics, a New York-based biotech group, and GenFit, its French rival.

Shares in Intercept jumped 13 per cent last week after the company said it had redesigned a late-stage clinical trial of its drug, obeticholic acid, in a way that would make it easier for the study to succeed.

Following discussions with the US Food and Drug Administration, Intercept said the trial must now show that the drug can either resolve Nash or improve “fibrosis” — the medical term for liver scarring. Previously, the company would have had to prove the medicine could do both.

Intercept also said it would be able to analyse the data once it had enrolled 750 patients, rather than 1,400, after the process of recruiting patients to the trial took longer than the company had originally expected.

The changes reflect the flexibility of regulators at the FDA, who want to encourage drugmakers to invest in researching medicines for the untreatable condition before the number of sufferers increases rapidly.

However, the sluggish recruitment to Intercept’s trial is indicative of one of the biggest hurdles facing Nash drugmakers: the disease is very difficult to diagnose.

The vast majority of people do not realise they have the “silent” disease, which only causes symptoms such as jaundice, fatigue and weight loss in the very late stages.

Doctors sometimes suspect a patient has Nash when they have elevated liver enzymes in their blood or when they spot abnormalities during ultrasound scans.

But the only surefire way of diagnosing the disease is to carry out a liver biopsy — a complex, invasive procedure that requires an anaesthetic. In rare cases, the biopsy can cause bleeding that requires a transfusion or surgery.

Many patients are reluctant to undergo the procedure, which poses a difficulty not just for recruiting them to clinical trials, but also convincing people in the real world to get tested for the condition.

Drugmakers are hoping that less invasive tests will be developed that use biological clues or “biomarkers” to diagnose the disease without the need for a biopsy.

“We really believe that there will be big breakthroughs that will make it much easier for physicians to diagnose the disease and subsequently to utilise the drugs in development,” says Dr Nicholson.

One option is a simple breath test, according to Dr Ilan Yaron, chief medical officer of Tiz Pharma, a London-based biotech group that is about to start testing its Nash drug in humans. “The FDA does not recognise these tests yet, but it will in time,” he says.

However, even if drugmakers find a reliable, non-invasive way of diagnosing the disease, there is no guarantee that Nash sufferers will rush to start taking their medicines.

The disease progresses slowly and many sufferers will die of something else; given the lack of symptoms, people could be wary of taking a drug for life when they feel perfectly well.

“I’m sceptical as to whether this is actually needed for most people,” says Ronny Gal, analyst at Bernstein. “You’re essentially trying to put into chronic treatment forever a broad population that will probably never develop the most serious form of the disease.”

Cash-strapped healthcare systems could resist paying for the drug in all but the sickest patients, predicts Mr Gal. These number approximately 1.5m in the US.

For that reason, he believes the market for Nash drugs will be much smaller than many others on Wall Street are expecting, and is pencilling in peak sales of $8bn for what he describes as a “midsize condition”.

“If you take all the people with advanced Nash and ask how many people will end up becoming seriously ill in their lifetimes, the number could actually be very small,” he says.