Amarin investors are getting decidedly twitchy. And with good reason. The Irish company this week goes before a US Food and Drug Administration panel seeking approval to increase tenfold the market for its prescription-grade purified omega-3 fish oil.

At present, its Vascepa drug – the first and, to date, only product the company is marketing – is approved as a therapy for people with severe triglycerides (excessive blood fats that increase the risk of cardiovascular disease).

That’s a four million-strong patient market, but the early enthusiasm for the company’s stock has been dented by its failure to secure “new chemical entity” status from the FDA, granting additional protection, and by the lack of either an alliance with or takeover by a larger pharma to market the drug.

Tomorrow, it will look to persuade an FDA panel that it should be allowed treat people with somewhat lower levels of triglycerides, who are taking a cholesterol-lowering statin. Success would open a 40-million strong market – over one in five of all Americans – and fundamentally change the outlook for the business.

By the same token, failure to win backing for a broader dispensing regime would represent a major setback for the company. At best, it would become a very long-term market play; at worst . . .

Such is the volatility surrounding the stock that even deferral of a final decision until December will knock back a share price that has already taken a beating in recent days.

Although it was trading up marginally at US lunchtime yesterday, Amarin has shed almost 30 per cent of its value in the past five trading days on the Nasdaq exchange following a precipitous collapse on Friday as the briefing document for the panel meeting was released by the FDA.

The fall was blamed on what the drug’s supporters have said was an unnecessarily negative assessment of that report. In fairness, the report seems fairly balanced – whatever about the coverage of it – but nor is it a ringing endorsement of the therapy.

There are several pointers suggesting that Amarin may secure approval, if not necessarily straight away, to bolster its market position. But the wary recovery of the stock’s value yesterday says that even its proponents are not entirely sure of their ground.