GlaxoSmithKline has been fined £37.6 million for striking deals with potential rivals to delay their launch of generic copies of its drug for anxiety disorders and depression.
The Competition Market Authority's Michael Grenfell told ITV News that GSK's actions had inflated the price of the antidepressant Seroxat and cost the NHS and taxpayer.
GSK argued its actions had brought down the cost of medicine for the NHS.
The drugmaker said it strongly disagreed with the regulator's fine for market abuse and was considering grounds for appeal.
Other drug companies involved were also fined smaller amounts, bringing the total fines issued by the CMA to £45 million.