Increasing blood viscosity (thickness) can be caused by abuse of which ergogenic aid?

Synthetic Erythropoietin

Explanation:

EPO as mostly called increases the rate of erythropoesis, that is production of red blood cells.This increases oxygen supply in the body, especially to the muscles cells. it is mostly used by athletes to improve their performances.

However as much RBC is  produced, the cells clog together making the blood thick or viscous .

This lead to circulatory load on the heart which exerted much force higher than normal cardiac  pressure  to pump  into systemic circulation.

This may cause heart heart attack.

the accumulated  aggregated RBC can also  lead to clotting. if this  blocked blood supply to the brain, it may lead to STOKE.