Like other steroids, testosterone may be harmful in excess, especially when extra is taken without proper medical supervision; acne , highly oily skin, growth of some cancers, difficulty sleeping, and other complications are associated with abnormally high testosterone levels. However, a deficiency can also be harmful, and will have an impact on libido , energy, and maintenance of general health. Most human bodies are able to regulate hormone levels naturally, requiring no medical intervention. Some older men participate in androgen replacement therapy programs when their natural testosterone levels decline in order to maintain their younger state of health and vigor.
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. 
If a young man's low testosterone is a problem for a couple trying to get pregnant , gonadotropin injections may be an option in some cases. These are hormones that signal the body to produce more testosterone. This may increase the sperm count. Hedges also describes implantable testosterone pellets, a relatively new form of treatment in which several pellets are placed under the skin of the buttocks, where they release testosterone over the course of about three to four months. Injections and nasal gels may be other options for some men.