A lot of genuinely bad pens exist on the market, though what I mostly use come from a major pharmaceutical manufacturer originally built for dispensing insulin. Feel pretty confident these hold their accuracy over time.
Checked calibration on literally every pen I own using a precision gold scale accurate to 0.001g, and they're all right on target.
Stick to pens exclusively for a handful of reasons.
Reason one, I filter absolutely everything, and buying a cartridge runs cheaper than a fresh sterile vial each time.
Reason two, pens create noticeably less medical waste overall. My sharps container gets emptied maybe a tenth as often now.
Reason three, ever since I started the copper peptide, needles previously pushed through a rubber vial cap started stinging more. Could switch to Luer lock setups with needle swaps instead, but circles back to reason one.
Reason four, my vision isn't great anymore, and a large printed '10' on a pen dial is far easier to read than tiny syringe markings.
Reason five, tracking how much liquid remains is simple thanks to a surprisingly accurate window on the pen's side.
Reason six, I'm genuinely clumsy pulling peptide from a vial, either trapping air pockets or coming up short on syringe volume.
Reason seven, easier to carry around.
Reason eight, honestly just a fan of well-made gadgets.
Reason nine, keeps my fridge shelf tidier overall.