Blood will break down both the cocaine and the heroin due to the presence of nonspecific esteraaes.
A classic teaching biochemistry lab experiment is measuring enzyme activitivity in blood (spit nowadays because people are afraid of blood). If the enzyme still is active it is able to cleave a synthetic molecule that gains color, and you measure the color change to get an idea of the enzyme activity.
But, the enzymatic activity of a small amount of blood in a syringe will never degrade the drugs at a higher rate than the when the drugs are in the body.
The time taken to shoot wouldn't give the blood a huge amount of time to react with the drugs.