"There is a positive association between ADHD [attention-deficit hyperactivity disorder] and suicidality in both sexes and in all age groups."
So said the results of the systematic review published by Balazs & Kereszteny  (open-access available here) who surveyed the recent peer-reviewed literature on the topic between 2011 and 2015. Reporting results based on findings included in 26 papers, authors concluded that suicidality - ideation, behavior, attempts, and suicide - seems to be an over-represented facet following receipt of a diagnosis of ADHD and that "early recognition and treatment of ADHD - either as a comorbid condition or as a main diagnosis- and the co-occurring psychiatric disorders, can play an important role in the secondary prevention of suicide."
These are important findings. Alongside the various research results suggesting that ADHD may set someone up for a variety of 'future adverse outcomes' (see here) there are potential lessons to be learned for a whole host of labels/conditions where ADHD is part and parcel of the clinical picture. Given the core blogging material included on this site and the realisation that autism and ADHD can very much co-occur (see here) I'm inclined to suggest that the Balazs/Kereszteny findings are particularly pertinent to the autism spectrum and the issue of suicidality being over-represented when it comes to the diagnosis (see here for example). Indeed, when one talks about suicidality in the context of the autism spectrum and how one might mitigate any enhanced risk (see here), ADHD as one of several over-represented labels following autism may represent another important consideration. And that also goes for another over-represented but perhaps 'atypically presented' condition potentially following a diagnosis of autism .
To close, although suicide (ideation and/or attempt) is a complicated and often very individual process, that's not to say that general intervention(s) might not be useful ...
 Balazs J. & Kereszteny A. Attention-deficit/hyperactivity disorder and suicide: A systematic review. World J Psychiatry. 2017 Mar 22;7(1):44-59.
 das Neves Peixoto FS. et al. Bipolarity and suicidal ideation in children and adolescents: a systematic review with meta-analysis. Ann Gen Psychiatry. 2017 Apr 21;16:22.
 Riblet NBV. et al. Strategies to prevent death by suicide: meta-analysis of randomised controlled trials. Br J Psychiatr. 2017. April 20.
Balazs J, & Kereszteny A (2017). Attention-deficit/hyperactivity disorder and suicide: A systematic review. World journal of psychiatry, 7 (1), 44-59 PMID: 28401048