Veins are the blood vessels which are much closer to the body surface so it is quite easy to inject required chemical in that vessel. Also veins carry blood to the heart so any chemical injected into the veins is carried to heart where it mixes with other blood and then carried by arteries from heart to the body parts. Generally injected fluid is in concentrated form so if we inject it in arteries,there may be possibility that concentrated fluid damage the tissue.

Veins are more easily accessible, many being superficial, no high pressure so no risk of the blood backing up into the infusion fluid which would defeat its purpose, and after removing the needle no need to press firmly for 5 minutes to prevent a hemorrhage from taking place.

Under special circumstances however we do inject substances into an artery, taking all precautions needed to e.g. prevent blood spill or bleeding afterwards, e.g. injecting blood vessel dilatory drugs likeMethyldopain conditions where arteries spasm causing lack of blood mostly in hand/feet fingers/toes inRaynaud's disease, in inoperable tumors sometimes blood clots or other solids are injected into the artery that feeds the tumor, thus causing the tumor to (partially) die off because of the lack of blood feeding it so slow the growth and make it more sensitive to chemotherapy and radiation treatment seeCatheter Embolization, and then in rare cases chemotherapy is administered into the artery feeding the tumor which might be in the pancreas, liver, brain, eye