Sharing of syringes by injection drug users is a principal means by which the human immunodeficiency virus (HIV) is spread. Some have suggested that distributing syringes that are difficult to reuse (DTR) would slow the spread of HIV. The authors developed a simple mathematical model that describes how changes in the numbers of DTR syringes or regular syringes consumed over the course of a fixed number of injections affect the proportion of injections that are potentially infectious and, thus, the transmission of HIV. It suggests that increasing consumption of either type of syringe will reduce the proportion of potentially infectious injections, but that, per syringe added, the reduction is always greater if a regular rather than a DTR syringe is added. Similarly, introducing a certain number of DTR syringes and simultaneously reducing the consumption of regular syringes by the same number will increase, not decrease, the proportion of infectious injections. DTR syringes are more expensive than regular syringes, so there is little justification for substituting them for regular syringes.