It’s not so much the chemo that harms the veins, it’s the constant puncturing, often on the ‘good veins’, it builds up small amounts of scar tissue - not huge but then again neither are the veins! Small needles help, as does making sure you are really well hydrated before the stabber has a go. I was having three tests a week at one point, on one arm, so yes it can get difficult. My hand veins are larger so I didn’t mind going there although it invariably bruised. Now I only have tests once in a while, so not much difficulty but the scar tissue is still there.