Every day patients or carers log on to newsgroups, chatrooms and forums to exchange information. Health is one of the most important subjects of internet traffic, third only to computers and holidays.

Potentially, the opinions exchanged are of great value to manufacturers of all kinds of products, not just the drug companies. They can provide early warning of problems, such as side-effects in the case of drugs, and enable companies to correct faults before they damage sales. Just as valuably, they contain lots of information about rival products.

This is made possible by a new web tool called iReputation, which has been developed by the online brand marketing company NetRank. The system monitors the internet for keywords — such as the name of a drug — trawling every relevant site every few minutes.

It picks up a large range of opinions, sorting them into a trend graph. From time to time the graph shows sharp peaks as a new issue appears, which alerts brand-owners. Generally companies that use the system, including the mobile phone manufacturer Motorola, employ a person to sift the data.

John Straw, chief executive officer of NetRank, said that there were at least 5,000 postings a day to health newsgroups and 10,000 to forums. “We can pick up every single reference to a drug name, then display the results graphically. But it’s up to the human manager to make judgments.”

He declined to say which of the main drug companies had subscribed to the service, which costs £1,000 a month for each brand name. The tool could be especially valuable in Britain and other EU countries where laws prevent drug companies from making direct contact with patients.

Mr Straw believes that the system has something to offer drug regulatory agencies in post-marketing surveillance. At present, side-effects of drugs are recorded on Yellow Cards filled in by doctors and sent to the Medicines and Healthcare Products Regulatory Agency. But the system picks up problems only when patients report them to their GPs.

• Internet-users who chat with friends using MSN and AOL Instant Messenger services are being targeted by hackers spreading viruses to empty internet bank accounts. Lycos, the internet service provider, said last night that millions of people who use the messaging windows could be at risk.

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