Though there is little evidence to establish that these drugs cause suicidal thoughts and behaviors, it's a distinct possibility they may contribute to the risk.

The warning is already in place for one ADHD drug, Strattera (atomoxetine), which had the risk information added to its prescribing information.
"New information has emerged since to suggest that the risk of suicidal thoughts and behaviors may apply to all other ADHD drugs," Health Canada officials said.
They also said that there is little evidence to establish that these drugs cause suicidal thoughts, but it is possible that they may contribute to the risk.
“It is also important to note that people with ADHD may already have a slightly increased risk of suicidal thoughts and behaviours,” officials added.
Source-Medindia