A group of UK drugs wholesalers are seeking an injunction to stop the world's biggest pharmaceutical firm changing the way it supplies medicine.
Pfizer provides about 15% of prescribed drugs in the UK
From Monday, Viagra-maker Pfizer will sell its prescription drugs through just one medical wholesaler, Unichem.
Eight wholesalers have launched the legal action, claiming that the agreement breaches competition law.
US-based Pfizer said the wholesalers were bringing the High Court action only to protect their own interests.
The Office of Fair Trading is already investigating whether Pfizer's move breaks competition rules.
The Dispensing Doctors Association (DDA) - which represents family GPs prescribing medicines - is supporting the wholesalers in their injunction bid against Pfizer.
DDA chief executive Dr David Baker said there were fears within the wholesaler community that the supply of medicines throughout the UK would be at the mercy of big drugs companies.
"If a single supple chain breaks down there will be nowhere else for us to get Pfizer drugs which is worrying," he told BBC Radio Five Live.
Dr Baker also said that if Pfizer products could only be bought from Unichem, there would be a loss of competition and choice.
"Our members are very unhappy about the loss of choice and competition among wholesalers who do compete very hard for our business," he said.
Prescription charges are fixed by the NHS. But Dr Baker added that as NHS prices are based on what prices are paid for the drugs, prices paid by patients could go up if a single wholesaler was able to charge more.
Pfizer, whose products also include the anti-cholesterol drug Lipitor, said it would defend the legal challenge.
"The wholesalers are acting to protect their commercial interests," it said in a statement.
"The legal challenge may jeopardise supply to patients because wholesalers have been encouraging pharmacists not to sign up with Unichem."
The wholesalers bringing the appeal are AAH Pharmaceuticals, Phoenix Medical, Mawdsley Brookes, Munro Wholesale Medical Supplier, Maltby & Sons, Norchem, PIF Medical Supplies and Sangers (Maidstone).