Financial conflicts of interest in systematic reviews (e.g. funding by drug or device
companies or authors' collaboration with such companies) may impact on how the reviews
are conducted and reported. To investigate the degree to which financial conflicts
of interest related to drug and device companies are associated with results, conclusions,
and methodological quality of systematic reviews. We searched PubMed, Embase, and
the Cochrane Methodology Register for studies published up to November 2016. We also
read reference lists of included studies, searched grey literature sources, and Web
of Science for studies citing the included studies. Eligible studies were studies
that compared systematic reviews with and without financial conflicts of interest
in order to investigate differences in results (estimated treatment effect and frequency
of statistically favourable results), frequency of favourable conclusions, or measures
of methodological quality of the review (e.g. as evaluated on the Oxman and Guyatt
index). Two review authors independently determined the eligibility of studies, extracted
data, and assessed risk of bias. We synthesised the results of each study relevant
to each of our outcomes. For meta‐analyses, we used Mantel‐Haenszel random‐effects
models to estimate risk ratios (RR) with 95% confidence intervals (CIs), with RR >
1 indicating that systematic reviews with financial conflicts of interest more frequently
had statistically favourable results or favourable conclusions, and had lower methodological
quality. When a quantitative synthesis was considered not meaningful, results from
individual studies were summarised qualitatively. Ten studies with a total of 995
systematic reviews of drug studies and 15 systematic reviews of device studies were
included. We assessed two studies as low risk of bias and eight as high risk, primarily
because of risk of confounding. The estimated treatment effect was not statistically
significantly different for systematic reviews with and without financial conflicts
of interest (Z‐score: 0.46, P value: 0.64; based on one study of 14 systematic reviews
which had a matched design, comparing otherwise similar systematic reviews). We found
no statistically significant difference in frequency of statistically favourable results
for systematic reviews with and without financial conflicts of interest (RR: 0.84,
95% CI: 0.62 to 1.14; based on one study of 124 systematic reviews). An analysis adjusting
for confounding due to methodological quality (i.e. score on the Oxman and Guyatt
index) provided a similar result. Systematic reviews with financial conflicts of interest
more often had favourable conclusions compared with systematic reviews without (RR:
1.98, 95% CI: 1.26 to 3.11; based on seven studies of 411 systematic reviews). Similar
results were found in two studies with a matched design, which therefore had a reduced
risk of confounding. Systematic reviews with financial conflicts of interest tended
to have lower methodological quality compared with systematic reviews without financial
conflicts of interest (RR for 11 dimensions of methodological quality spanned from
1.00 to 1.83). Similar results were found in analyses based on two studies with matched
designs. Systematic reviews with financial conflicts of interest more often have favourable
conclusions and tend to have lower methodological quality than systematic reviews
without financial conflicts of interest. However, it is uncertain whether financial
conflicts of interest are associated with the results of systematic reviews. We suggest
that patients, clinicians, developers of clinical guidelines, and planners of further
research could primarily use systematic reviews without financial conflicts of interest.
If only systematic reviews with financial conflicts of interest are available, we
suggest that users read the review conclusions with skepticism, critically appraise
the methods applied, and interpret the review results with caution. Financial conflicts
of interests and results, conclusions, and quality of systematic reviews Patient treatment
practices are often based on clinical research. Systematic reviews are a core type
of such clinical research. When several similar studies (i.e. studies investigating
the same research questions using similar methods) have been conducted, these can
be identified and analysed in a systematic review. Systematic reviews thereby summarise
existing studies and provide an overview of a specific research field. Thus, systematic
reviews may have a major influence on decisions about patient care and it is essential
that such reviews are trustworthy. Sometimes, systematic reviews are funded by companies
with a financial interest in the review's results and conclusions, for example because
they produce a drug or device investigated in the review. At other times, systematic
reviews are carried out by researchers with a personal financial interest in a specific
result, for example when the researcher acts as a consultant for the company producing
an intervention that is assessed in the review. These financial conflicts of interest
may impact on how systematic reviews are conducted and reported. Our Cochrane Methodology
Review focuses on financial conflicts of interest related to drug or device companies
in systematic reviews. Our primary aim was to investigate the degree to which systematic
reviews with financial conflicts of interest present review results and make conclusions
that are more favourable than systematic reviews without such financial conflicts
of interest. Our secondary aim was to investigate the degree to which systematic reviews
with financial conflicts of interest differ in methodological quality from systematic
reviews without such financial conflicts of interest. We found 10 studies comparing
systematic reviews with and without financial conflicts of interest. Based on two
of these studies, we found no evidence of a difference in review results between systematic
reviews with and without financial conflicts of interest. Based on seven studies,
we found that systematic reviews with financial conflicts of interest more often had
conclusions favourable towards the experimental intervention (risk ratio (RR): 1.98,
95% confidence interval (CI): 1.26 to 3.11). Also, based on four studies, systematic
reviews with financial conflicts of interest tended to have lower methodological quality
(RR for 11 dimensions of methodological quality spanned from 1.00 to 1.83). Our analyses
suggest that when systematic reviews have financial conflicts of interest related
to drug or device companies, they are of lower methodological quality, and have more
favourable conclusions. However, it is not clear whether this derives from actual
differences in the review's results or the over‐interpretation of those results. Based
on our findings, we suggest that people who use systematic reviews, including patients,
clinicians, developers of clinical guidelines, and planners of future research, could
primarily use systematic reviews without financial conflicts of interest. If such
reviews are not available, we suggest that users are especially cautious when they
read and interpret systematic reviews with financial conflicts of interest.