The explosion occurred on Monday night at Arenol, the US manufacturer which is the sole supplier of the active ingredients in Adderall and DextroStat, Shire's products for treating attention deficit hyperactivity disorder in the US. Shire shares plunged 86p to 393.5p as the company said it had enough supply of the products in stock to meet demand for the next three to four months.
However, the company also warned it was "unlikely" that it would be able to find another source for the ingredients before the current stock ran out.
"This will have an adverse effect on Shire's financial position," the company said, though it added that it could not quantify the potential damage.
In the six months to last June, the two products accounted for 63 per cent of Shire's revenues.
Rolf Stahel, Shire's chief executive, said: "We are very concerned about this unfortunate accident and will do everything reasonably possible to avoid stock-outs and to minimise inconvenience to patients." The company is seeking an alternative manufacturing facility.