Been injecting 2mg of the copper peptide daily for around ten weeks now. Use the extra-diluent method (mix 3ml into the vial to reconstitute, then combine that with another 3ml in a bigger container, then transfer into cartridges for my pen). Since I'm running a pen setup, I'd rather not cycle it between room and fridge temperature repeatedly, so I just inject it cold straight away. Have to go slow and then rub the area afterward a bit. Some days nothing happens, other days, like today, there's a mild annoying burn, more irritating than actually painful. Also get red bumps a handful of times weekly. Wish I had zero reaction since I don't intend to stop this anytime soon, might eventually look into adding a numbing agent to see if that helps.
Bottom line, plenty of options exist to take the edge off that sting, so it really comes down to how committed you are and whether you feel the payoff justifies it.