I went through this. The specialty pharmacy claimed only the ones they sent could be used but I think they were limited by the study. My PCP prescribed BD Nano Ultrafine 4mm x 32G and said it made no difference. The Rx was sent to my local drug store.

I also could not find the original 8mm x 31 G prescribed for Tymlos. It seems only specialty pharmacies have them. The specialty pharmacy that sent me my Tymlos will only provide for one month.

Those of us who are ramping up or taking less than a full dose will need more needles! My PCP said the 4mm is fine. Here is some info on needles. I have not noticed and leakage and since I am very thin, these are actually preferable. Judging from the side effects today they are working well 🙂
https://www.verywellhealth.com/diabetes-and-pen-needles-1087295#Needle%20Sizes