Common drugs for depression and Parkinson’s can sway people’s moral judgments about harming others, according to research that raises ethical questions about the use of the drugs.

The study found that when healthy people were given a one-off dose of a serotonin-boosting drug widely used to treat depression they became more protective of others, paying almost twice as much to prevent them receiving an electric shock in a laboratory experiment. They also became more reluctant to expose themselves to pain.

The scientists also found that the dopamine-enhancing Parkinson’s drug, levodopa, made healthy people more selfish, wiping out the normal tendency to prefer to receive an electric shock themselves, while sparing those around them.

Molly Crockett, a psychologist at the University of Oxford who led the work, said the finding that a single exposure to the drugs had such a noticeable impact on behaviour challenged the idea that we have stable moral values.

“Patients [taking these drugs] are tracked in terms of how their symptoms improve, but not necessarily in terms of how their behaviour changes,” she said. “In the treatment of Parkinson’s, some patients go on to develop compulsive gambling and compulsive sexual behaviour. The drugs have consequences that reach out into the world beyond the patient.”

She added that it was unclear whether the effects seen in the study would be replicated in patients. An alternative possibility is that the drugs could bring the behaviour of patients “back to baseline” by stabilising their psychological state.

“The central message is we need to have more research into how these drugs affect behaviour, both in healthy people and in people taking them for disorders,” she said.

In the study, published in the journal Current Biology, 175 participants took part, with 89 assigned to receive the anti-depressant citalopram or a placebo and 86 given either levodopa or a placebo.

The participants were also randomly designated as “deciders” or “receivers” and anonymously paired up. All participants were given mildly painful electric shocks matched to their pain threshold so that the intensity was not intolerable. Deciders were told that shocks to receivers would be at the receiver’s own pain threshold.

Deciders went into a room alone with a computer terminal, and each took part in around 170 trials. For each trial, they had to choose between different amounts of money for different numbers of shocks, up to a maximum of 20 shocks and £20 per trial. For example, they might be offered a choice of 7 shocks for £10 or 10 shocks for £15. Half of the decisions made in the trials related to shocks for themselves and half to shocks for the receiver, but regardless of who received the shocks, the deciders would get the money.

On average, people given a placebo were prepared to pay around 35p per shock to prevent harm to themselves and 44p per shock to prevent harm to others. Those on citalopram, the seratonin-based drug, were far more harm-averse, willing to pay an average 60p per shock to prevent harm to themselves and 73p per shock to prevent harm to others. Overall, they delivered around 30 fewer shocks to themselves and 35 fewer shocks to others than those on placebo.

People given levodopa, however, were not willing to pay any more to prevent shocks to others than to prevent shocks to themselves. On average, they were prepared to pay approximately 35p per shock to prevent harm to themselves or others, meaning they delivered on average 10 more shocks to others during the experiment than the placebo group.

“The dopamine drug made people more selfish,” said Crockett. “Most people show this pattern where they think it’s worse to harm other people than to harm oneself. That’s abolished by the drug.”

The researchers suggest that in future it may be possible to give people a simple test to assess whether their decision-making behaviour has been radically altered by a drug, as well as asking them about whether their mood and symptoms have improved.

“We’re not transforming someone from a healthy person into a criminal or anything like that,” Crockett said. “But in aggregate we make decisions multiple times a day and they can shape our lives.”