Valium has a high therapeutic index unless it's combined with other drugs.
A friend of mine died years ago from a mixture of injected Demerol and Valium.
Problem is that a junky is familiar with monitoring for a possible overdose smoking, injecting, or snorting, Especially with the fist two, they just have to monitor themselves or others for a short time and they're good to go, An oral drug like Valium takes longer to kick in and gives that extra respiratory suppression later on... oops.
With my friend, his cohorts kept an eye on him for a while to make sure he was OK after the shot and then left him to sleep in the car while they went in to play pool. The Valium kicked in later and they came out to find him dead.