What DBlue says is the same for me on Zoladex, except I ended up with two to start with, so I have one spare all the time, which although probably not intentional, seemed like a good idea across brexit. (I did read on another forum that the injection can go wrong, and another one will be needed, and some people keep a spare for that reason.) I can't make the (12 weekly in the case of Zoladex) appointments more than 4 weeks in advance, which is a slight annoyance. The nurse is very insistent that they won't give you the injection even 1 day early. Zoladex is injected into the fat under the skin of the abdomen. They record which side was done, and alternate.
However, I'm a bit unusual at my surgery for some unknown reason, and this is not how it normally works for their Zoladex patients, as most patients don't get a prescription. Instead, the surgery keeps a stock of Zoladex and uses that. However, since I do have a repeat prescription setup, I use that rather than the surgery's stock, so I'm not dependent on someone else having stocked up. (Probably one of the staff set up the repeat prescription unaware of their normal practice.)
Out of interest, does anyone do their own Prostap or Zoladex injections? I saw on a US forum that some people do their own Zoladex at least.