The pharmacy chain launched a review earlier this year with pitches taking place in July.
The account is believed to be one of Lexis' most lucrative. MD Jason Gallucci said: 'After four-and-a-half years, it is a shame that the relationship has ended. That's agency life, and life goes on.'
Sources said Red was now in final negations with Boots but the firm's UK head of PR Una Mullan insisted the pitch process had not been completed.
The Red Consultancy healthcare MD Pat Pearson declined to comment.