The 2000 RSA described an interesting phenomenon in Delhi and Kolkatta. Many people who engage in injecting drug use do not remain 'injecting drug users'. Instead, they go back to 'chasing' (or inhaling). The identification of 'injecting drug users', for the purposes of eligibility for sublingual buprenorphine, has been a complicated issue for a number of the partners. This is because a person who has experience in the high risk behaviour of injecting drug use is not necessarily an 'injecting drug user'.

In Delhi, the RSA identified that 27% of the opiate using population were currently engaged in injecting drug use. However, twice that number, i.e. a total of 54% of the opiate using population, had engaged in injecting drug use at some point in their lives. Sharan's field workers in Delhi reported a very similar observation. Figure 4 illustrates this phenomenon by showing a 'shadow' in the overall population of opiate users who have experience in injecting drug use, without currently engaging in the activity.