Half of doctors in hospital and general practice have been concerned about a colleague's abilities, a General Medical Council survey suggests.
The public and doctors were asked how they viewed regulation
The poll of 411 doctors found that 93% of those with worries had taken some form of action, such as talking to a senior colleague or employer.
However, 10% of GPs and 4% of hospital doctors had taken no action.
One in five of doctors who expressed concerns made reference to possible alcohol problems.
The GMC survey found doctors also raised concerns about communication skills, team-working skills and general ability and clinical knowledge.
A quarter of the GPs based their concerns on a belief that their colleague was severely overworked and suffering from stress.
When doctors did not raise concerns, the main reason was concern about the impact on their own career and relationships with peers.
Others said they believed the situation should be resolved by a senior colleague, or that they did not know who to report their concerns to.
'Good communication key'
A second poll of over 900 members of the public found they had more faith in the regulatory system than doctors.
Three quarters of members of the public said they had confidence in the way doctors were regulated, compared to 50% of GPs and 52% of hospital doctors.
Fewer than 10% of members of the public said they were "not at all confident" in the GMC, with half mentioning family experience as a factor in their views.
The results mark a significant change in attitude compared to a 2003 survey, where patients dubbed the GMC a bureaucratic "doctors' club".
Those doctors who were unhappy with the regulatory process tended to say the GMC did not treat doctors fairly, or was biased.
But an even lower number - less than a third - expressed any confidence in how they were managed by employers., who were seen as too focused on targets, not understanding medicine and funding problems.
When asked about what made a good doctor, nine out of 10 members of the public said giving good advice and treatment, closely followed by good communication skills.
Doctors mostly thought that recently trained doctors had good communication and interpersonal skills.
Sir Graeme Catto, President of the GMC, said: "We are heartened by the public expression of confidence in regulation and by the support from doctors of the principle of professionally led regulation.
"It is an important duty of doctors to raise concerns, when they have them, about a colleague and we are glad that the vast majority who see a problem endeavour to do something about it.
"However, we are concerned to ensure that those actions are followed through in the interests of patients.
"This means it is important that the GMC as a national regulator works closely with employers to ensure that this happens."
Alastair Henderson, deputy director of NHS Employers said: "The GMC's survey paints an encouraging picture of the willingness of doctors to pursue concerns about the ability of their colleagues.
"But it highlights the need for employers, the GMC and the professions to work together to ensure there are clear and well understood processes for handling concerns about doctors.
"We have issued guidance to GPs to remind them of their processes for raising concerns and we will continue to reinforce this.
"We are also continuing to support employers to develop their whistle-blowing procedures in trusts."