MAPS: JAMA: Twin study fails to prove 'gateway' hypothesis.

[Peter Webster <vignes@xxxxxxxxxx>]

From: Andrew Byrne <ajbyrne@xxxxxxxxxxxxxx>
Subject: Twin study fails to prove 'gateway' hypothesis.
Date: Tue, 10 Jun 2003 10:11:06 +1000

Australian researcher in JAMA lead article.

Escalation of Drug Use in Early-Onset Cannabis Users vs Co-twin
Controls. Lynskey MT, Heath AC et al. JAMA 2003 289:427-433

Dear Colleagues,
Twin studies can be informative in causation theories.  These authors state
that in addition to having close or identical genetic make-up: ".. twin
pairs, having been reared in the same household, would be expected to be
highly concordant for environmental experiences."  Thus most twins are
exposed to alcohol, tobacco and other drugs at much the same age.

In their lead item in JAMA, Lynskey et al. find that for the exceptional
minority of twin pairs (~300 out of 4000) in whom 'cannabis use before age
17' was discordant, that subsequent reported drug abuse/dependency was 2 to
5 times more prevalent in the early cannabis users.  The authors find that
this association lends weight to causation while admitting it is 'not
possible to draw strong causal conclusions' of the 'gateway' theory.  It is
intriguing that they would address causation when this is a retrospective,
cross-sectional study, a design which is not able to determine causation.

Since twins who used cannabis in the same year were eliminated, conclusions
based on their similarities of upbringing must be guarded.  These twins
demonstrated at least one major difference in their environment and/or
decision making on at least one occasion during adolescence.  Whatever
caused this may also explain the higher reported rates of other drug use,
quite independent of any theoretical 'chemical priming' or 'gateway' effect.

These results are all derived single follow-up telephone interviews with an
unknown party over matters relating to illegal drug use, child sex abuse
and other personal issues up to 15 years earlier. Some may have chosen to
(falsely) deny childhood cannabis use and then to also deny adult abuse or
dependency.  Others may have had faulty recollection for such distant
events, making the findings less secure.

A certain minority of young people use hard drugs prior to using cannabis
(around 1 - 2% from household surveys).  Such subjects should be of
considerable interest to those addressing the so-called 'gateway'
theory.  Lynskey et al. however, having found that up to 17 of their
subjects used hard drugs before being exposed to cannabis, chose to exclude
them from their study.

Another problem with this study is that the drug abuse/dependence findings
are so high that they may indicate an atypical sample.  For 'any illicit
drug abuse/dependence' the prevalence was 33-48%; for 'alcohol dependence'
it was 30-43% ['non cannabis by age 17' group first percentage followed by
'early users group' %].  Alcoholism is only thought to affect around 5% of
adult males in the general population.

The authors state that their findings "..were consistent with early
cannabis use having a causal role as a risk factor for other drug use and
for any drug abuse or dependence." But further, they state: "While the
findings of this study indicate that early cannabis use is associated with
increased risks of progression to other illicit drug use and drug
abuse/dependence, it is not possible to draw strong causal conclusions
solely on the basis of the associations shown in this study."  A 'causal'
link between early cannabis use and later hard drug use remains unlikely on
balance (National Academy of Science review) - and it is hard to imagine
that a study of this nature could clarify the issue, no matter how well it
was performed and analysed.

An associated editorial by Kandel teases some of the matters out while
still seeming to assume that all cannabis use is problematic and needs to
be discouraged by any effective means.  Her own work from 19 years ago
showed the strong association between alcohol/tobacco and illicit drug use.

Kandel indicates three factors necessary to prove the gateway hypothesis:
(1) sequencing, (2) association and (3) causation.  As she points out, the
third is the hardest to prove.

In discussing the modern calls for prescribed cannabis, Kandel states that
there is no empirical knowledge on whether medical cannabis will lead to
problems.  But cannabis products were widely prescribed in the first half
of the 20th century without apparent problems arising (eg. tincture of
cannabis).  She says that it is a 'curious phenomenon' that morphine used
for medical purposes 'does not lead to addiction'.  Could it be equally
'curious' that medical cannabis does not do so either?

And unlike Lynskey et al., Kandel does not address the known non- chemical
associations between cannabis and hard drug use.  Having found from
personal experience that drug education about cannabis was unreliable,
young people may then reason that information about heroin and cocaine
being dangerous is also unreliable. Similarly, having broken the law on
cannabis, they may then have less compunction about breaking laws relating
to other drugs. There is at least anecdotal evidence that some heroin
addicts first used heroin when their dealer could not supply cannabis,
amphetamine or other drugs of current choice.  To his credit,
Lynskey  writes: ".. access to cannabis use may provide individuals with
access to other drugs as they come into contact with drug dealers. ...
[Dutch decriminalization of cannabis] may have been partially successful as
rates of cocaine use among those who have used cannabis are lower in the
Netherlands than in the United States."  [One wonders what criteria the
authors would need for 'complete' success of Dutch cannabis policy!]


Comments by Andrew Byrne ..

Citations: Lynskey MT, Heath AC, Bucholz KK, Slutske WS, Madden PAF, Nelson
EC, Statham DJ, Martin NG. Escalation of Drug Use in Early-Onset Cannabis
Users vs Co-twin Controls. JAMA (2003) 289:427-433

Kandel DB. Does Marijuana Use Cause the Use of Other Drugs? JAMA (2003)
289; 4: Editorial

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   Dr Andrew Byrne,
   Medical Practitioner, Drug and Alcohol,
   75 Redfern Street, Redfern,
   New South Wales, 2016,
   Australia
   Tel (61 - 2) 9319 5524  Fax 9318 0631
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

author of: "Addict in the Family" and
"Methadone in the Treatment of Narcotic Addiction"
http://www.csdp.org/addict/
http://www.drugpolicy.org/library/byrne_contents_methadone2.cfm


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