Essential in life, Google may soon play a part in death. Earlier this month the company added an option to its Google Health site allowing users – so far only in America – to state their final wishes. "One of the most important documents you may want to store and share in Google Health is an advance directive," announced the Google blog. "An advance directive allows you to determine your end-of-life wishes so that your family and doctor can honor them if you get sick and are unable to communicate. The decision to sign an advance directive is an important and personal one, and Google Health now makes it a little bit easier."

Some people will find this sinister. A company that began by creating a brilliant search tool is straying into the most intimate of areas. Google has grown by being good at what it does, but also through what it knows. It tracks search histories, tailors advertising to email, and agglomerates content from every source it can get access to, fulfilling its corporate mission "to organise the world's information and make it universally accessible and useful". To that, Google might add profitable, since the way to internet riches is to know the details of users' lives.

In the Times yesterday David Davis, the former shadow home secretary, turned on his party over reports that British patients might be encouraged to use Google Health and its counterparts, rather than persist with the expensive and much-delayed NHS database. "Google is the last company I would trust with data belonging to me," he wrote. Rather than increase personal control over medical data, he argues that it would end it. Google is not subject to ministerial orders, or European regulation. It promises never to sell advertising alongside medical information, but it has done deals with US pharmacy chains under which these groups are encouraging their 100 million customers to lodge their prescription records with Google Health.

The Conservatives, whose independent review on medical IT reports soon, say there is no prospect of NHS records being handed over to Google in bulk. The party argues that it is simply preparing for an open source world, where individuals will manage and share data more effectively, and cheaply, than government ever can. If people want to use Google Health, or Microsoft HealthVault, they should be allowed to, on their own terms.

But for individuals to be empowered, they must first be protected. Data is only guarded by the promises of the organisations that hold it. Users can protest if the terms of their contracts are changed, but there are no central rules and no central control. For some, that is the attraction. But do not mistake this for a right to privacy.

• This article was amended on Tuesday 28 July 2009. We said that Google had done deals to get access to the prescription records of 100 million American patients. In fact, Google has reached agreements with some US pharmacy chains under which these groups are encouraging their 100 million customers to lodge their prescription records with Google Health. This has been corrected.