The demand for NHS Direct's online service now outstrips calls to its phone helpline as a "symptom checker" mobile phone app notched up more than one million downloads in six months.

The number of digital transactions - about 7.3 million per year - is significantly more than that of calls to the 08454647 number, which stands at about 4.5 million.

"More people now access NHS Direct's services online than they do over the phone and it is our aim to continue to provide our services in places that patients and the public will find useful and convenient," said Roger Donald, NHS Direct's associate director, multi-channel (acting).

"The mobile app is a more discreet and less embarrassing way of seeking health advice for sensitive issues if you're in a public or crowded place."

The app, which has been reviewed by NHS local, was released for Android and iPhone smart phones at the end of May 2011. In its first week it climbed to number one on the iTunes chart of top free apps. By the end of November, the number of downloads had topped one million.

The symptom checker helps users to self-diagnose an illness or health problem by giving them access to reliable healthcare advice straight to their smart phone.

Free to download, it uses the same logic and questions as the health and symptom checkers found on the NHS Direct website.

The app covers a wide range of health problems, including dental pain, diarrhoea and vomiting, abdominal pain, rashes, back pain and burns.

Advice is also available about how to relieve symptoms associated with specific conditions such as 'flu and hay fever.

There is also the opportunity to obtain more specialist advice on issues such as mental health, contraception, sexual health matters and pregnancy problems.

Because the app is linked to NHS Direct's phone service, if a further assessment is recommended, people can submit their contact details and an NHS Direct nurse advisor will call back. 

For iPhone users, the symptom checker is available from iTunes; for Android users, the app can be obtained from the Android Market.