Drug companies are bombarding GPs with promotional materials and inducements, campaigners say.
Which? surveyed 200 doctors
A poll of 200 GPs by consumer group Which? found they received four visits per month on average from drug reps.
They also received five promotional mailings about new drugs a week, and inducements to attend conferences.
Which? said it raised questions about drugs patients were being given by GPs, but a drug industry spokesman said it was vital doctors were kept up to date.
The Which? survey found one in four of the GPs questioned had been sponsored to attend a conference, seminar or training event in the UK in the last 12 months and 5% had been sponsored to attend an event abroad.
In just one month, one GP was offered nine conference places and 13 meals, and received nine visits from drug reps, 10 letters, 21 leaflets, two patient information booklets and one training DVD.
This amounted to 22 companies contacting her about 31 drugs.
Nearly half of the doctors told Which? there was a lack of information from independent sources, while just 7% said they trusted the information they received from drug firms.
Neil Fowler, of Which?, said: "When you get a prescription from your GP, you want to know you've been prescribed the right drugs, not drugs produced by the company that spent a lot of money on promotion and inducements.
"We want to see more sources of independent information on drugs so that GPs can make balanced decisions, more limits on the marketing of drugs, and transparency about funding."
Richard Ley, spokesman for the Association of the British Pharmaceutical Industry, said: "I make no apologies for the fact that pharmaceutical companies are in close contact with doctors about new medicines.
"It is right and proper that they inform GPs about new medicines, and how they might benefit their patients, so that doctors are kept up to date."
Mr Ley said the ABPI had a strict code of practice to ensure ethical dealings with doctors.
"If Which? has evidence of where that code of practice has possibly been breached, then we would want to see it," he said.