Thirty-one gauge needles do the job well enough. The containers come with a vacuum, so I push in air matching the amount of liquid I pull out. There might be some resistance left. For mixing the powder, twenty-seven gauge works better for me. I looked at several videos online showing how to prepare peptides when I first tried it.
I had trouble drawing up the bacteriostatic water for my retatrutide because the needle was too fine at 31 gauge. I figured out later that using separate needles and a single syringe would be easier, but I managed anyway. The toughest bit was actually drawing and injecting the peptide itself since it resisted even for just 3 cc, and I ended up going through three needles. Any ideas?