The GPC said many GPs would immediately fall foul of new GMC prescribing rules on unlicensed and off-label prescribing because they are too 'vague' and prioritise licensed drugs.
If the GMC takes action against GPs who breach the guidance it would 'decimate' the profession and raise the NHS drugs bill, GP leaders warned.
GPC prescribing lead Dr Bill Beeby said: 'The implications are significant.'
In 2011, the GMC set out plans to ease restrictions on prescribing unlicensed or off-label drugs. The BMA had welcomed the move and said it would help GPs make better use of NHS resources.
Previously, doctors could only prescribe unlicensed drugs when they would better serve the patient than licensed alternatives.
Draft guidance published in June 2011 planned to relax this rule and allow doctors to prescribe unlicensed drugs where they were 'as safe and effective' as licensed ones.
However, the GMC has now reversed its plans after taking legal advice on EU regulations.
Under the new rules, published on Thursday, doctors can only prescribe unlicensed or off-label drugs when a licensed alternative is unavailable or inappropriate for a patient. It also said GPs cannot prescribe unlicensed drugs on the basis of cost.
The new rules, Good practice in prescribing and managing medicines and devices, will come into force on 25 February.
Dr Beeby said: 'The section on unlicenced medicines will be a problem. It seems to remain sufficiently vague to allow interpretation and, therefore, the use of off-license medication for individual patients, while at the same time suggesting that "licensed medicines" will take a priority.'
He said the rules suggested that cheaper generic medicines should not be used off-license when a licensed product exists, even when there is strong scientific evidence for efficacy.
Dr Beeby warned: 'Today, I suspect that a large percentage of GPs will fall foul of this guidance and the rest will do so over the next week as they prescribe amitriptyline in chronic pain rather than pregabalin.
'If the GMC does not act, then its failure to do so will set a precedent. If it does, then the GP workforce could be decimated. Drug budgets will then certainly rise, and medicines management might be powerless.'
Niall Dickson, GMC chief executive, said: 'Safe prescribing is at the heart of good medicine and it a skill that doctors must develop and keep up to date throughout their careers. This new guidance tightens the current rules on prescribing and addresses the challenges doctors face in this complex area.'
The new GMC prescribing guidance also sets out how:
- Doctors should avoid prescribing for themselves or their families unless it is an emergency or lives at risk
- They should 'encourage' patients to be open about the use of illegal drugs, alternative remedies and medicines obtained on the internet
- Use of unlicensed drugs must be re-assessed each time a doctor reviews a patient's medicines - for instance, use of antipsychotics in dementia
- Medicinal products including Botox, Dysport or Vistabel cannot be prescribed over the phone or online.
Stephen Whitehead, chief executive of the Association of the British Pharmaceutical Industry (ABPI), said: 'Decisions on the prescribing of unlicensed medicines are never easy, and this is why it is so critical to get this guidance right. The GMC has listened to stakeholders and taken an open and constructive approach to engaging with us. The new guidance reaffirms that patient need should be at the heart of clinical decisions.'