The effect is similar to smoking's effect on a person's heart. The blood vessels within the penis are constricted, which allows for decreased blood flow. This lessened flow has a negative effect on the elasticity of the penis and prevents it from achieving its full length once erect. Studies have shown that over time, these effects can reduce the overall length by as much as 1 cm!
According to one researcher, smoking has more direct harmful impact on the penis than it does on the heart!