The 'Flying Without Wings' singer spoke about anxieties over appearance on ITV's Good Morning Britain as part of its #SelfieEsteem campaign that aims to boost confidence.
Nicky said self-esteem can affect males just as much as females, pointing out his own experience as part of Westlife.
"In those kind of crazy Westlife years it is something that you can't avoid, you see yourselves everywhere on magazine covers, on TV regularly and then you know you kind of think, 'I need to do that, I need to change, I need to train a little bit'," he recalled.
"It's kind of unavoidable and I think people probably see it as a female thing predominantly and it's not really, I think guys do have those insecurities as well."
Nicky went on to say his teeth caused him self-esteem issues, particularly when manager Louis Walsh suggested he have them fixed.
"I remember the early days it was my teeth that were all over the place. It's lads banter amongst yourselves when people are saying 'ah your teeth are this'... it's a bit of banter. But Louis used to always say to me 'you've got to get your teeth done, you've got to get your teeth done' and I suppose that resonated with me then and I went and got a brace.
"In the midst of the Westlife fame I saw Tom Cruise on one of the red carpets wearing the white train track braces and I thought, you know what? If the biggest star in the world can go and do that then why not. A lot of the Westlife earlier years I would have smiled with my mouth closed while the rest of the boys were blinging teeth.
The #SelfieEsteem campaign – which is also backed by the likes of Josie Gibson and Rebecca Ferguson – encourages viewers to upload a selfie to social media on their first attempt to take it with absolutely no filters.