She has left the firm to take up a directorship with Hill & Knowlton's office in Sydney, Australia.
It is understood that AstraZeneca has yet to reach a decision on a replacement for Tigar, and has appointed Janine Nolan acting head of comms in the interim.
No-one at AstraZeneca was available to comment on the details of Tigar's departure or the timescale for appointing a successor.
Tigar joined AstraZeneca in February 2000 with a mandate to construct her own PR team following the merger of Swedish firm Astra and UK firm Zeneca the previous year (PRWeek, 18 February 2000).
That recruitment process was completed in October last year after the appointment of 15 staff in 19 months (PRWeek, 12 October 2001).
Last month AstraZeneca reported an eight per cent year-on-year rise in profits, making £660m profit in the three months to June, with sales also rising by eight per cent.