Flash is leaving his position as business unit director of the agency's health education division next month after more than five years at Virgo.
He joins Tonic in December as managing director of its newly created UK healthcare business division. He will report to Tonic co-founder Oliver Parsons.
Flash will be responsible for developing the agency's existing UK client work and adding to the division with new clients. He will also handle the agency's professional relations activity, which includes market access work.
Tonic's existing clients include Johnson & Johnson, AstraZeneca and GlaxoSmithKline.
Tonic CEO and co-founder Scott Clark said: 'Tonic's UK client base has traditionally taken up 20 to 25 per cent of our business. We are already considered strong in our international work, but we now want to win more UK business.'
He added: 'In the UK there are so many regulatory changes that there is a sales prevention mentality. While we respect the ABPI code, we want to embrace more positive change with a more positive attitude, and we think Neil has the outlook we look for and will hugely benefit the agency.'
Flash joined Virgo four months after its launch in 2003. He reported to agency co-founders and joint managing directors Angie Wiles and Sarah Matthew.
Flash previously worked with Wiles at Cohn & Wolfe as an associate director. He left shortly after Wiles when she launched Virgo with Matthew, who was MD of Sugar Consulting.
Wiles said: 'Neil will be sadly missed, but it's the right move for him to go out and try something on his own.'
She added that Flash's role was now being split in two and that replacement appointments were expected to be made very soon.
Virgo took its second Healthcare gong at this year's PRWeek Awards last week for its work with existing client Nurofen Express.