The CMA said the company had charged unfairly high prices for phenytoin sodium, an epilepsy drug, for over 4 years.
Pharma firm Flynn was also charged £6.7 million as part of the CMA investigation.
The regulator said the firms de-branded the drug, previously known as Epanutin, meaning it was no longer subject to price regulation and the firms could choose what prices to set for the life-saving drug.
Pfizer charged the NHS prices between 780% and 1,600% higher than previously over those four years, according to the CMA.
CMA boss Andrea Coscelli said: “These firms illegally exploited their dominant positions to charge the NHS excessive prices and make more money for themselves – meaning patients and taxpayers lost out.
“Such behaviour will not be tolerated, and the companies must now face the consequences of their illegal action.”
The fine follows a £90 million fine imposed by the CMA in December 2016 after finding that that the two firms had breached competition law. The decision was challenged and brought to the Court of Appeal in March 2020, after which the CMA reopened its investigation.
A Pfizer spokesperson said the company disagreed with the decision and would be appealing against it.
Pfizer’s profits more than doubled to hit $22 billion in 2021 after its coronavirus vaccine was granted emergency approval by health regulators. The jab is thought to have saved millions of lives worldwide.