Methylphenidate and Autism Ranking:

There are a number of limitations to all of the research studies published to date. For example

Eight of the studies used single-case designs, that is, they did not have a control group of participants who did not receive the intervention.
Some of the single case design studies used relatively weak, non-experimental methodologies. For example the study by Nickels et al, 2008 and the study by Stigler et al, 2004 used retrospective chart reviews.
One of the randomised controlled studies (Kim et al, 2017) was non-blinded, meaning the participants and researchers may have known which participants received which dosage of methylphenidate.
Some of the randomised controlled studies (such as Pearson et al, 2013) did not adequately describe the randomisation process. This means that the participants may not have been properly randomised between the groups.
Some of the randomised controlled studies (such as Handen et al, 2000) did not adequately describe the blinding process. This means that the participants and researchers may not have actually been blind (unaware of which participant received which intervention).
Some of the randomised controlled studies (such as the RUUP, 2005) may have been subject to performance bias. That is, the participants may have been able to guess whether they were taking the medication or the placebo because of the side effects of the methylphenidate.

All of the randomised controlled studies except two included fewer than 30 participants. Four of the single-case design studies included fewer than 20 participants and one (Birmaher et al, 1998) included only nine participants.
Some of the studies were restricted to specific groups of participants. For example, Quintana H. et al, 1995 looked only at primary school age children diagnosed with autistic disorder.
Some of the studies examined participants with a range of conditions, only a proportion of whom were autistic. For example, Simonoff et al (2013) included a large number of participants with ADHD, only some of whom were autistic..

Some of the studies investigated a range of stimulant medications, making it difficult to know if any reported effects were produced by methylphenidate and/or by another stimulant. For example, the study by Nickel et al, 2008 included participants who were taking methylphenidate, dextroamphetamine, mixed amphetamine salts, pemoline and/or methamphetamine.
Most of the studies ran for relatively short periods of time (four to eight weeks).
Some of the studies examined participants who received one or more other interventions at the same time as they received methylphenidate. For example, Ghuman et al, 2009 included participants who were receiving one or more medications (such as prednisone, albuterol, ﬂonase, atropine and melatonin) and/or other therapies (such as special education services, speech and language therapy, occupational therapy, physiotherapy and behaviour therapy).
Some of the studies (such as Research Units on Pediatric Psychopharmacology Autism Network, 2005) used a cross-over design with no washout phase between the methylphenidate and the placebo.

Some of the studies (such as Quintana et al, 1995) said that they used a range of outcome measures but did not provide data for all of those measures.
Some of the studies did not break down outcome data by specific groups of participants. For example, the study by Simonoff et al (2013) did not provide separate outcome data for the autistic participants.
Most of the studies did not identify if methylphenidate had any beneficial effects in the medium to long term (six months or longer).

Very few of the studies appeared to involve autistic people and/or parents and carers in the design, development and evaluation of those studies.
Many of the studies were funded by the manufacturer and suppliers of methylphenidate. The researchers involved may therefore have been biased towards the intervention, however unconsciously.

For a comprehensive list of potential flaws in research studies, please see Why some research studies are flawed.

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