Obviously no doctor would ever approve, but I have been reusing dispossible syringes and later disposable needles ever since the switch from glass syringes 45 + years ago. These days I usually use the same needle for the entire contents of a pen
Crucially I have never come to any harm.
I have only chosen to replace a needle occasionally ( other than starting a new pen) after I have had to inject in some potentially unsanitary venue.
I have just checked my current box of 100 needles ( still with about 20 left) and note it was prescribed in mid September 2023 , so it will probably end up lasting a year, though I will be asking for next box in August.
PS I try to keep a small supply of everything separate from my main supplies for just such an emergency.