What to do if pain in my left arm when i smoke.?

Artery tightening: Smoking can cause spasm of any artery in the body among which those supplying the arm. I would advise if you would stop smoking with the help of a smoking clinic if you have been failing on your own. The pain you are feeling in your arm could be sign of an underlying tightness in the arm artery. An imaging of the artery with ulltrasound can show you the abnormality.

How about stopping?: There is an old joke - "Doc, it hurts when I do this." "Don't do that!!" Smoking causes intense constriction of arteries, more so in some very susceptible people, and you could be experiencing angina or heart pain. If it is only when you smoke cigarettes, I would be very concerned and see your doctor immediately for more complete evaluation. AND STOP SMOKING!! Get into a cessation program asap.