Parkinson’s medication blamed for sexual offences

Adrian Carter and Wayne Hall, from the Neuroethics group at the University of Queensland Centre for Clinical Research, Australia

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The medication that provides significant relief from debilitating motor disturbances in people with Parkinson’s disease appears to cause a range of psychiatric disturbances that are as distressing and difficult to treat as the motor symptoms they aim to relieve.

Parkinson’s disease is usually treated with dopamine replacement therapy (DRT). This involves daily dosing with either levodopa (a precursor to the neurotransmitter, dopamine) or dopamine agonists (such as pramipexole and ropinirole) that mimic the effects of dopamine in the brain. The aim of DRT is to reduce the effects of the loss of dopaminergic neurons in specific regions of the brain involved in controlling bodily movement. However, dopamine is also a key neurotransmitter in a range of cognitive processes from executive control and memory to motivation and bonding. It is perhaps unsurprising that many Parkinson’s patients experience adverse psychiatric and cognitive side-effects from taking large doses of dopamine every day. 

Parkinson’s patients can experience severe anxiety, depression and mania and have a higher risk of suicide. A significant minority of Parkinson’s patients treated with dopamine replacement therapy will also develop impulsive and compulsive behaviours that appear to be caused by their medication. These include pathological gambling and hypersexuality, and compulsive eating and shopping. In rare cases, patients have committed criminal offences.

In a recent case in Tasmania, a former MP was convicted of having sex with an underage girl. The defence lawyers argued that he had developed a compulsive sexual disorder as a result of his Parkinson’s medication, the dopamine agonist, pramipexole, that caused him to commit the offence. His lawyers argued that since starting the medication, the defendant had sought the services of prostitutes on 506 occasions over a two year period. The prosecution accepted the expert medical evidence to this effect provided by the defence.

The defendant pleaded guilty to the offense but was given a 10 month wholly suspended sentence on the grounds that the medication had caused the defendant’s behaviour and he was at low risk of repeating the offences having stopped taking the medication.

There has been a similar ruling in a case in the UK . A Parkinson’s patient was found with almost 8000 pieces of child pornography on his computer. He was convicted of child pornography offences but given an absolute discharge because the offences were deemed not to warrant punishment (although he was required to sign the sex offenders registry). Interestingly, while almost all the photographs were downloaded after the individual was prescribed DRT, a single image had been downloaded before commencing the medication, raising questions about whether this represented some latent predisposition. The judge dismissed this as an aberration.

How well based are these decisions? What sort of evidence is there that DRT can cause these types of compulsive criminal acts?

There is reasonable evidence that these medications play at least a contributory causal role in DRT-induced compulsive behaviour (for a more comprehensive review of this evidence, see our papers in Neuroethics and Addiction): Firstly, compulsive disorders, such as pathological gambling, occur significantly more often in PD patients receiving DRT than in the general population. Over 85% of these compulsive behaviours occur in people taking dopamine agonists – cases are rare in those on levodopa alone. Secondly, there is a close temporal relationship between starting these medications and the onset of the compulsive behaviour: the behaviour usually emerges after patients start taking DRT or after large increases in dose; and the behaviour typically remits after the medication is stopped or the dose is significantly reduced. Such an effect is also biologically plausible: DRT acts upon the same pathways in the brain affected by drugs such as amphetamines. Parkinson’s patients receiving DRT also show similar learning deficits to individuals who are addicted to stimulant drugs (for example see Cools et al.).

The vast majority of Parkinson’s patients treated with DRT do not develop compulsive or impulsive behaviours, or at least behaviours so noticeable that they come to the attention of the authorities. Why is it then that some patients treated with DRT develop compulsive behaviours that are criminal? And why do some people engage in compulsive behaviours that are criminal or harmful and not other more adaptive or constructive behaviours (there are reports of some individuals developing artistic interests and creative thinking following dopaminergic medication)?

A large study of Parkinson’s patients treated with DRT (n=3,090) identified a number of factors that make someone more likely to develop these compulsive behaviours. These included: developing Parkinson’s and being prescribed medication at a younger age (a factor that might be explained by the fact that those who develop Parkinson’s at a younger age are more likely to be prescribed dopamine agonists, rather than levodopa, and at higher doses); and a personal or family history of addiction, drug use or other impulsive or compulsive behaviours.

These risk factors raise interesting and unresolved questions: If someone falls into one of these higher risk categories, are they less responsible for their behaviour? Does the existence of a latent desire to engage in criminal or harmful activities that an individual had successfully resisted until taking DRT make them more or less responsible for their behaviour? These are important questions that have yet to be addressed by the courts or discussed in the legal, clinical or ethics literatures. The cases that have come before the courts illustrate that this is more than an interesting philosophical question about agency and free will; it is an issue that judges and juries will need to grapple with.

It is also not clear whether individuals treated with DRT endorse their behaviours or identify with their new interests. This will probably depend upon the way in which society views these behaviours and on whether they cause harm or discomfort to others. For instance, in the cases above, both of the individuals convicted of sexual offences disavowed their actions and identified the medication as the cause. This is not necessarily true of all behaviours that occur after commencing on DRT. In one case report, a male patient developed a strong interest in anal sex that caused great distress to his wife. He claimed that he had these desires before taking DRT but was too embarrassed to act on them: the medication allowed him to “realise these desires”. His interest in engaging in these sexual behaviours ceased after a change in his medication, and he later expressed regret at his earlier behaviour that he attributed to his medication.

There has been no research so far to determine whether people prescribed DRT believe that these changes in behaviour are authentic expressions of themselves. Similar issues have arisen in studies of the users of other drugs that affect the dopaminergic system. Ilina Singh and colleagues interviewed adolescents treated for ADHD with Ritalin (a drug that also increases dopaminergic stimulation) and their parents. They found that the attribution of authenticity to actions depended on whether the behaviour was seen as positive or negative. For example, a child’s bad behaviour was often attributed to a failure to take their medication, whereas success on the sporting field was more likely to be attributed to the child. This reflects a common human characteristic to take personal credit for our successes and blame circumstances for our failures.

The occurrence of compulsive behaviours in Parkinson’s patients treated with DRT may have implications for our understanding of the addictions more broadly. There has been considerable debate about whether, and if so how much, addiction impairs a person’s ability to desist from using drugs. Some have questioned whether addiction in fact exists.  Compelling arguments have been provided for various answers to these questions (see here, here and here). It would seem, however, that the increased risk of ICDs in patients treated with DRT challenges wholly sceptical accounts about the effects of chronic drug use on behaviour. PD patients are especially interesting because as older adults, often without histories of addiction, they comprise a population otherwise at low risk of developing addiction. Yet a substantial minority of these patients develop compulsive behaviours after taking DRT; some even develop a dependency upon their medication (referred to clinically as dopamine dysregulation syndrome). This adds some weight to the argument that the chronic use of drugs can produce changes in brain function that make it much more difficult for individuals to desist from using drugs or engaging in problem gambling despite wishing to do otherwise. The fact that only a minority of those who take DRT develop these behaviours indicates that the effects of dopaminergic drugs on the brain are not the whole story, but they may prove to play a theoretically and ethically interesting contributory role.