The Huddersfield-based firm, which employs 425 people, generated sales of £66.2m last year from a stable of brands that includes Covonia cough medicine and pain relief lotion Radian B.
Stada, which makes medicines for Parkinson’s disease and stomach complaints, as well as antibiotics and suncream, said the deal will make a “positive contribution” to profits from the beginning of next month.
The two companies agreed to hold exclusive takeover talks earlier this month as the German group sought to strengthen its position in the “especially important” UK market.
Stada said the acquisition will allow it to “introduce prescription-free branded products from the comprehensive group portfolio in the British market in the future”.
The firm recently unveiled a 10 per cent rise in first-half sales to €974.3m (£831.1m) and chief executive Hartmut Retzlaff said he was confident of further growth in the second half and beyond, with a target of €2.15 billion in group sales for 2014.
Thornton & Ross was founded in 1922 and is chaired by Jonathan Thornton, the third generation of the family to work in the business.