In general, it's incredibly easy to OD with street drugs especially, as purities greatly vary. With IV, the risk increases greatly, because the margin of error can be so low and the BA is so high. It's also a lot easier to OD when one is not consistently using (which sounds odd but it's true). People with a habit, who are using everyday and know their tolerance and are getting a consistent batch of dope, have a much smaller chance of OD'ing because their body is so used to it, and they can handle much more. Where as somebody who is chipping, never quite knows where their tolerance is, and their body is not used to the drug, therefore it's effects will be much more pronounced and unpredictable.
But, no matter how you often use, it's important to remain vigilant and careful. A lot of OD's happen from recklessness and carelessness. Be smart, do test doses, don't go overboard, and try to keep some semblance of control.
