The results of a single study considered in isolation can be misleading.
If a single study shows that patients receiving a treatment did better/worse than another group, it does not mean that is the final answer.
Conclusions become more reliable the more times they can be replicated. The conclusions of one study may be contested by that of a similar study with differing results. This could happen because the comparisons were different between the studies, or because they were small studies with few animals or events.
To ensure you are using the most reliable information it is preferable to have multiple evidence sources showing similar results.
BEWARE if a claim is supported by a single study/piece of evidence.
REMEMBER: Check for opposing results or extra support for the studies you use in decision-making. Reviews, when conducted effectively, can be useful collections of similar studies.