Final answer:
The irreversible physical effect of performance-enhancing drugs is the thickening of the blood, alongside other nonreversible consequences like impaired heart function and testicular atrophy.
Step-by-step explanation:
Most physical effects of performance-enhancing drugs are reversible, with the exception of thickening of the blood. The use of these substances carries serious health risks which are often significant and nonreversible. Particularly, anabolic steroid use can lead to irreversible consequences such as impaired heart function, testicular atrophy, prostate enlargement, and other serious health issues like cardiovascular disease and aggressive behavior. These effects persist even after the cessation of drug use and could have lifelong impacts on the individual's health.