My pharmacy only had 8mm needles in stock and since I have enough 6mm needles to last until next Monday I asked the pharmacy if they could get 4 or 6mm needles in for me, which they can. The reason I asked them to get 4 or 6mm needles is because my educator said I shouldn't use above 6mm. But, I realised when I got home today after the pharmacy that I don't know why I couldn't use 8mm needles. I'm not going to go against the advice of my educator and the pharmacy said they'll keep 4, 5 and 6mm in stock for me (they were very nice) so that's cool. What's the reason for using smaller length needles?