I'm no airline pilot but have the greatest respect for those of you who are and have to make these decisions quickly. Going on from what Wiggy says, I remember an AAIB report from a few years ago involving a flight en route from the States to the UK. Shortly after takeoff the first officer felt unwell with a headache and talk a tablet out of his pocket (thinking it was parcetomol) only find out soon after that it was Co-proximol type drug which made him ill. The captain decided to press on and then almost landed on the worng runway at EGKK due to a late runway change and the presure of work on him that it caused. For those interested it was June 1997
AAIB Bulletin No: 6/97 Ref: EW/G96/12/1 Category: 1.1