Often in a Cochrane Review, there is only one included study. If this is the case, authors should consider presenting the results in a table, rather than in a series of forest plots. Avoid drawing strong conclusions from single studies at unclear or high risk of bias.
The Cochrane Handbook (Chapter 11, Section 22.214.171.124) states:
“Forest plots should not be generated that contain no studies, and are discouraged when only a single study is found for a particular outcome. To diplay outcomes that have been investigated only in single studies, authors can use a forest plot using a subgroup for each outcome (ensuring that the option to pool the data is disabled). Otherwise results of single studies may more conventiently be presented in an additional table”