Let's be clear, very clear; there are side-effects of testosterone use, but this is largely due to supraphysiological amounts. If you supplement with exogenous testosterone for the purpose of replacement therapy, the odds of incurring problems are almost non-existent; you're simply replacing what you no longer produce. Where adverse reactions become a problem is when doses get too high; of course, fortunately most can tolerate a very high level, and if side-effects do begin to mount, for the healthy adult male they can be combated; in-fact, with responsible use they can be altogether avoided.
So, how does one ensure that testosterone levels remain in balance? Some doctors suggest that monitoring testosterone levels every five years, starting at age 35, is a reasonable strategy to follow. If the testosterone level falls too low or if the individual has the signs and symptoms of low testosterone levels described above, testosterone therapy can be considered. However, once testosterone therapy is initiated, testosterone levels should be closely monitored to make sure that the testosterone level does not become too high, as this may cause stress on the individual, and high testosterone levels may result in some of the negative problems (described previously) seen.