The side effects of testosterone is usually dose dependent and the time period you stay on for. The side effects are well known and documented by now and we can discuss this at great length. Its become a bit of a boring topic to discuss and repeated in every article you read on testosterones. But lets mention them in summery. The list is as follows – aromatizing in the body to estradiol (estrogen) that can cause water retention, body fat gain and gynocomastia. You can combine an anti-estrogen like Nolvadex or Clomid to generally take care of most of these side effects. These anti estrogens can be alternated with a more effective aromatize inhibitor like Arimadex but only as an alternative and not for continues use (see the article under Sustanon to study the cholesterol and cardiovascular issues with aromatizing inhibitors). Oily Skin, acne and body/facial hair growth in men and woman is evident but also dose related and varies from individual to individual. Males who have a genetic predisposition to hair loss may experience accelerated male pattern balding. Testosterone does not have hepatotoxic effects and liver toxicity is unlikely.
Testosterone is significantly correlated with aggression and competitive behaviour and is directly facilitated by the latter. There are two theories on the role of testosterone in aggression and competition.  The first one is the challenge hypothesis which states that testosterone would increase during puberty thus facilitating reproductive and competitive behaviour which would include aggression.  Thus it is the challenge of competition among males of the species that facilitates aggression and violence.  Studies conducted have found direct correlation between testosterone and dominance especially among the most violent criminals in prison who had the highest testosterone levels.  The same research also found fathers (those outside competitive environments) had the lowest testosterone levels compared to other males.