Dr Rosenthal, the director of the university's cognitive behavioural research and treatment program, said many drug addicts continued to suffer despite receiving the best known treatments. "As part of six months of counselling, during 12 sessions, the counsellor uses virtual reality to help the patient learn to confront cravings directly in a simulated world full of crack cocaine related cues," he said in an interview.

"When the cravings go up and no crack is smoked, after several minutes the cravings then go down. Once they go down we play a tone over and over again to the patient. This tone becomes conditioned to the experience of learning that the patient can have cravings, but not use." Mobile phone software is then given to patients so they can hear the tone and provide feedback on their cravings 24 hours a day. As a result of the virtual Pavlovian conditioning, the patient should eventually associate Dr Rosenthal's tone with the sensation of decreased craving. The goal is for patients to be able to control the cravings even when in high-risk environments.

Using virtual worlds for medical purposes has already proven successful for Australia's Inspire foundation, which launched an online role-playing game in September, designed to help young people overcome mental health problems. The game, Reach Out Central, was last week announced as the joint winner of the global "Why Games Matter: A Prescription for Improving Health and Health Care" competition, which looked at ways video games were transforming health care.

Made using virtual reality software called VR Worlds 2, Dr Rosenthal's cyber crack house could easily be mistaken for a video game. And for good reason, since it's based on the same gaming engine used for the first-person shooter classic Half-Life 2. Therapists are able to customise the virtual environment to suit individual patients, but the ultimate goal is to elicit cravings and then to eliminate them. Patients can be taken to locations including an abandoned crack house, apartment, motel, alley, restaurant/bar, middle class home and park.

"Starting with low cravings, the therapist guides the patient over time to increasingly more intense people, places and things," Dr Rosenthal said. "At the most intense level, patients may see multiple people buying, selling and using crack inside an apartment together."

While the study is ongoing, Dr Rosenthal said patients had told him the virtual world was highly realistic and effective at reducing cravings. Several asked if they could keep calling in from their mobile phones to hear the tone after they left the program. "If our data show that this approach is successful ... then I imagine we will have some interest in testing how to implement this treatment in the real world of community treatment programs in America, Australia and beyond," he said.