AstraZeneca shares are leading the losers on the FTSE 100 after the US Treasury department revealed that it forging new rules that will make it more difficult for companies to move their bases abroad to avoid tax.
The AstraZeneca stock price has tumbled by over 5% to 4348.50p by midday while rival GlaxoSmithKline was hit by 1.25% at the same time.
AstraZeneca was the subject of one of the most high-profile takeover attempts by US firm Pfizer earlier this year.
Pfizer admitted during a number of sessions in UK parliament that it snapping up AstraZeneca would be hugely beneficial for the company's tax payments.
"I am acutely aware that the future of the company is how to efficiently get products to the patients while also making sure we have good capital allocation. We are not the only company that has reduced on employment," said Pfizer boss Ian Read in May this year.
"[If the deal does go through] we see lower tax rates [for the company] and a flexible use of financial aspects."
The US pharmaceutical giant already operates 128 subsidiaries in tax havens, booking $69bn in profits offshore – the same amount it was looking to buy AstraZeneca for.