A few things first: 10 units only tells volume, not mass, unless the vial was reconstituted with exactly 1 ml of BAC. The heart-rate jump right away sounds like nerves, since retatrutide's resting-heart-rate effect doesn't hit that fast. The four-day chest tightness is more worrying if other symptoms were present. Was there injection-site redness, a rash, or tongue swelling? First step is checking whether the vial was tested and what the COA showed, plus how much BAC was added total.
New to this stuff, I tried 1 mg from a 10 mg vial using 10 units eleven days ago. The racing heartbeat in the first hour might have been nerves from finally going through with it, but the bigger problem was chest tightness that made breathing hard. Tightness eased after three or four days and breathing took another two days to settle. I still want to see results so I'm thinking about starting again at 0.5 mg for a few weeks. Does that sound okay, and has anyone else had similar sides?