Testosterone and anabolic steroids have been found to affect the central nervous system in laboratory animals and humans. Their locations of affect in the brain are closely linked to centres that regulate mood, sexuality and aggression (1).
People who use steroids in excessive doses often experience, during the using period or after it, mood disorders that meet the criteria of psychiatric disease categories such as depression, anxiety, psychotic reactions and cognitive deterioration (2).
Anabolic steroids affect the brain neurotransmitter systems.
It has been studied that steroids affect the brain serotonin and dopamine neurotransmitter systems.
Dopamine is a multi-function neurotransmitter participating in the regulation of mobility, learning, emotions, appetite and positive reinforcing effects. It is an important neurotransmitter for operating the reward system, so dopamine is essential for addiction development. Serotonin, in turn, regulates sleep patterns, movement, appetite, sexuality and emotions. According to some studies, it is also related to memory operations and sustainable concentration.
Among other findings in animal experiments we’ve found that an anabolic steroid, nandrolone, changes the dopamine system response to stimulating intoxicants. The release of intoxicant-induced neurotransmitters lessens, and so does the pleasure procured from using intoxicants. (3.4). In some studies, androgenic compounds have been shown to have direct activating functions for dopamine and serotonin release (5.6).
It is known that anabolic steroid abuse in high doses may impair a number of organs and functions, causing both physical and psychological illnesses. It is noteworthy that many of these symptoms are found to be long-lasting even after discontinuation of using these compounds (7.8).
Large doses have been demonstrated to induce programmed cell death in many cell types, including neuronal cells (9). This may result in irreversible changes in the nervous system. Anabolic steroid abuse has also been associated with psychiatric disorders and increased use of intoxicants.
We have sought to identify effects on the brain dopaminergic and serotonergic nervous system of an anabolic steroid, nandrolone decanoate, at doses that cause peripheral changes commonly sought-after by users, such as increased red blood cell synthesis.
We have been interested in seeing whether nandrolone alters the direct nerve chemical and behavioural effects of cocaine in laboratory animals. We have also sought to find out whether there are sustainable changes to nerve pathways in the brain caused by administering nandrolone, and after how long the changes will disappear. 
Effects of nandrolone on the nervous system are long-lasting
Our results definitely show that nandrolone has long-lasting effects on the brain dopaminergic and serotonergic nervous system. Although nandrolone levels apparently decreased and eventually disappeared completely in rat blood, nandrolone-induced changes were still detectable. 
It took about five times the administration duration period for the dopamine system to return to its initial level before administration. Serotonin system recovery lasted, in turn, six times the duration of the administration period. Anabolic steroid induced changes in the central nervous system’s dopamine and serotonin systems therefore seem to recover, but it takes a considerably long time compared to the duration of the administration period. 
In summary, the use of anabolic steroids can cause considerably long-lasting changes in the user’s brain neurotransmitter pathways. The serotonin neurotransmitter system often reacts the strongest to nandrolone administration. This supports the previously observed side effects, such as aggressiveness and mood changes, because decreased serotonin levels in the brain relate to the aggressive and uncontrolled behaviour of both humans and animals.
Our study results suggest that the breaks generally observed by users in between the administration periods are not long enough to recover the studied neurotransmitter pathways activity to their basic level. When a new administration period begins before the neurotransmitter pathways activity has been recovered, the new changes can be greater and the effects can last even longer than before.
PhD, Senior Researcher
National Institute for Health and Welfare