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Keywords:

  • Addiction;
  • dependence;
  • dopamine replacement therapy;
  • illicit drug use;
  • Parkinson's disease;
  • psychostimulant

Ambermoon et al. [1] have highlighted a phenomenon that has hitherto been reported predominantly as a syndrome of medication side effects, rather than dependence or addiction. While the compulsive and excessive use of dopamine replacement therapy (DRT) and associated behavioural and psychopathological features were first documented in the early 1980s, reports of this phenomenon have been confined largely to psychiatry, neuroscience and movement disorders publications. It was not until two decades later that compulsive DRT use was reported in the addiction literature [2], and only now that the potential utility of this syndrome as a model for psychostimulant addiction has been explored.

The debate over whether compulsive DRT use is a syndrome of dependence is reminiscent of those over the validity of other types of drug dependence. Historically, there has been a reluctance to attribute the compulsive, problematic use of a drug to dependence if it cannot be explained fully by the recognized neurobiological and behavioural models of addiction. Whereas the clinical manifestations of opiate addiction, i.e. a compulsion to use with a clear tolerance and withdrawal syndrome, are consistent with accepted theories of addiction, addiction to other substances has been less clearly understood and defined.

One such example is that of cannabis dependence, where there has been much debate as to the existence of a cannabis withdrawal syndrome. Despite mounting evidence for the existence of cannabis withdrawal, the nature of such a syndrome has not been well defined and there has been a lack of clear diagnostic criteria [3,4], with only relatively recent recognition of the clinical significance of cannabis withdrawal symptoms.

The road to recognition of a psychostimulant dependence syndrome has been similarly rocky. Despite evidence for the development of tolerance and compulsive use of cocaine, the absence of a reliable and clearly defined withdrawal syndrome, with reported withdrawal symptoms varying in nature and severity, has led to questions regarding the existence of a physiological dependence on cocaine [5]. Similarly, in the late 1990s, despite widespread use of amphetamine and reports of associated problems, there was still a paucity of evidence regarding the validity of an amphetamine dependence syndrome [6]. More recently, there has been much debate regarding the existence and nature of ecstasy dependence, which differs from that of other drug dependence with respect to the lesser relevance of a withdrawal syndrome [7].

The concept of tolerance and withdrawal is complicated by the course of Parkinson's disease (PD). The progression of PD is associated with increasing severity of symptoms, the alleviation of which may require higher doses of medication. Moreover, there is the added difficulty of disentangling symptoms of withdrawal from DRT from those of PD itself.

As noted by Ambermoon et al. [1], a further barrier to the acceptance of compulsive DRT use as a form of addiction has been reluctance to introduce the stigma associated with illicit drug use into a population that are using prescribed medications for the symptomatic relief of a debilitating disorder. A similar argument, however, could be made for the destigmatization of illicit drug use, given that self-medication of psychological pain is a well-recognized impetus for initiating and maintaining illicit drug use. Irrespective of the genesis of these behaviours, the outcomes are similarly poor for both types of users. Moreover, many pharmaceutical agents, such as pharmaceutical stimulants and opioid analgesics, are prescribed with the knowledge that they have a significant abuse and dependence liability. Should the dependence on these medications, if it eventuates, be regarded as something other than dependence purely because they were prescribed originally for a medical condition?

Ambermoon et al. [1] suggest that considering compulsive DRT use as a form of addiction will contribute to the improved understanding and treatment of these behaviours among PD patients, although there are wider clinical implications for the use of dopaminergic medications for other conditions. The compulsive medication use and associated behaviours seen among PD patients have also been documented among people receiving DRT for restless legs syndrome [8].

There are also implications for future research. While those who receive DRT to treat the symptoms of neurological disorders may differ from the typical illicit drug user, issues pertinent to illicit drug dependence may be of research relevance among this group. One such issue is the potential for the transition from use of dopaminergic medications to use of illicit psychostimulants. Are patients who develop compulsive DRT use at risk of engaging in drug substitution when they are unable to obtain their medication?

Although compulsive DRT use may be confined to a relatively small and specific population, and may arise from a genuine therapeutic need, it is nevertheless characteristically similar to psychostimulant dependence and warrants further examination within the addiction field. While summarized more eloquently by Ambermoon et al. [1], if it looks like a duck, swims like a duck and quacks like a duck, it probably is a duck.

References

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  2. Declaration of interest
  3. References