In minor but life-altering ways, drugs used to treat certain illnesses can play on the moral fabric of the brain, reveals a study.
Dopamine drugs make people selfish while those given anti-depressants tend to behave in ways protecting others from harm, shows research done by Molly Crockett, an Oxford University psychologist.
Compulsive gambling and sexual behaviour in patients with Parkinson's has been linked to the treatment, according to Crockett.
The drugs may not turn "a healthy person into a criminal" but they have potential life-altering effects by influencing our decisions, she told The Guardian while calling for more studies into the behaviour-changing effects of drugs.
Her study found healthy people given a dose of a serotonin booster drug used in depression became more reluctant to give other people an electric shock.
But this altruistic tendency disappeared among those given levodopa, a dopamine-enhancing drug used for people with Parkinson's.
The research is published in the journal Current Biology.
In the study, 175 people were given either anti-depressants, levodopa or placebos.
They were split into pairs with one person offered money for giving an electric shock to the other. Money could also be paid to prevent shocks.
Those given a placebo offered to pay an average of 35p to stop themselves receiving a single shock and 44p to stop someone else getting one.
This, explains Crockett, is the natural pattern where most people think it's worse to harm others than themselves.
But while those on the serotonin booster paid more to stop the other getting a shock, those on levodopa were not as considerate.