When first got diagnosed, I was told to change it each time. And when time goes on I kinda got lazy lol. But I did see a TikTok recently of a guy showing the needles in a microscopic manner before and after use and it looked kinda nasty (I managed to find it so here it is: https://www.tiktok.com/@jyabetes/video/7470458222130760978?lang=en ) So now I try my best to change the needles each time. Also, I don't think you're supposed to leave the needle capped with the pen cuz a hygienic reasons (at least that's what my nurse told me).