The value of Irish exports fell 5% in November due to a slump in shipments of pharmaceuticals and organic chemicals.

Official trade figures for November, the latest month for which statistics are available, show the seasonally adjusted value of imports rose 3%. November's trade surplus, the value of exports less the value of imports, fell 15% to just over €2.5 billion.

That's the lowest seasonally adjusted monthly trade surplus since August 2008.

The value of exports in November was down 7% on the same month in 2012. According to the Central Statistics Office the main drivers were "decreases of €572m in the exports of medical and pharmaceutical products and €158m in the exports of organic chemicals".

The vulnerability of Irish export figures to the ups and downs of the pharmaceutical sector is a recurring theme. The pharma sector dominates goods exports from Ireland, accounting for almost half of all merchandise exports by value.

Nine of the ten largest drug companies in the world have significant operations in Ireland. Six of the top ten best selling medicines which are due to come off patent protection between 2011 and 2016 are manufactured in whole or in part here.

As the patents on those drugs expire competitors are able to offer lower cost copycat products with same formulation. Price cuts reduce revenues for the patent holders and, therefore, impact on the value of Irish exports of those drugs or their active ingredients. The impact of the cluster of blockbuster drugs, with sales in excess of $1 billion a year each, coming of patent at the same time is known as the "patent cliff".

In a working paper last November the Department of Finance concluded that the effect of a decline in exports from the pharmaceuticals and chemicals sector could knock up between 2.7% and 5.1% off overall economic output (GDP) over a five year period.

The fall in exports from the sector is partially offset by the resulting fall in imports of raw materials.

The Department also notes that about 35% of the gross value of output from the pharma-chem sector is made up of payments by the Irish subsidiaries to their parent companies in the form of royalties and other payments.