You do not have to inject into the vein directly, when your needle passes through/catches the vein then sometimes some of the solution is drawn into the vein (remember veins take blood back to the heart so there is a negative pressure/suction which will draw in fluids if close enough), as it passes through your vessels around your chest you feel a coughing effect as if you have fluid on your lungs. No one really knows the exact reasons but the less that gets drawn in through a vein in one go the less the cough will be if at all. This will be related to Trens negative effects on the cardovascular system in my opinion, as the short breath although less intense can be quite simular
