A spokesman for the pharmaceutical giant said 246 permanent jobs out of the 672-strong team would be cut over the next four years at the site in Southdown View Way, East Worthing.
The company will be outsourcing some of the work done on the site.
It also plans to sell the Horlicks and MaxiNutrition brands in the UK, as part of a shift towards investing more into manufacturing respiratory and HIV medicines.
Roger Connor, president of GSK global manufacturing and supply, said: “We have a substantial manufacturing presence in the UK and continue to support the network with new investment of more than £140million in the next three years. At the same time, we have had to make some decisions which we know will cause uncertainty for some of our employees. We will do all we can to support them through this process.”
GlaxoSmithKline employs around 17,000 people across the UK, of which 5000 are in UK manufacturing operations.
A spokesman said none of the announcements made by the company resulted from the UK’s decision to leave the European Union.