If you work with someone like me who has adult ADD, there can be some challenges but for the most part it’s a workable situation. In some cases it can even be a better-than-normal situation, surprisingly.
I think that if the coworker is taking medication for his or her ADD, you won’t have any real problems with their distractability. The meds make such a huge difference, transforming the most distracted person into someone who is generally focused on their work.
You might have problems with the coworker’s impulsiveness, irritability, anger, and frustration. Those are common parts of the disorder. As someone who has struggled with these traits, I can say that they will come up from time to time. The meds also help with these though, in particular the irritability.
If your work has a creative or decision-making aspect to it, working with someone who has ADD can make things interesting for the better. People with ADD tend to come up with unusual, often random-sounding ideas. Your strange ideas are also more likely to be supported and believed in. That is because people with ADD tend to be less inhibited in their thinking.
If your work is administrative or requires a lot of planning and organization, it can be difficult to have a co-worker with ADD. The best way to make things go smoothly if you’re sharing planning & organizational duties with an ADD person is to try and handle more of the detail-oriented parts while letting him or her handle more of the person-to-person interaction or the thinking & deciding parts.
I’m not saying that this is exactly what you have to do, but it’s more of my list of suggestions if you’re already having a hard time working with your adult ADD co-worker and want things to go a little more smoothly. But if things are working out and you’re doing exactly the opposite of what I suggest, there’s no reason to change them.