It would be nice to think that he balances and deliberates before making his choice - but there are more dubious pressures at play.  All has been revealed by a drug company salesman who has just retired, and perhaps has a pang of conscience.

He writes: "It's my job to figure out what a physician's price is.  For some it's dinner at the finest restaurant, for others it's enough convincing data to let them prescribe confidently and for others it's my attention and friendship.  But at the most basic level, everything is for sale and everything is an exchange".

In the States there are 2.5 drug reps for each doctor, and the art of persuasion has become the science of 'detailing', as it's known.  Doctors' psychologies are assessed, while their prescribing habits are carefully monitored (the Food and Drug Administration happily sells on all prescribing data to the drugs companies).  Their weaknesses are targeted, and their vanities are massaged, our repentant drug salesman tells us.

There are usually at least 10 major drugs for every common condition.  So how does the doctor decide on the drug that's right for you?Even the doctor who initially refuses to 'play the game' can be won over.  With the right combination of humility and 'academic proof',  the most sceptical doctor will also start prescribing your drug.

The 'academic proof' comes in the form of studies and trials that are published in the prestigious medical journals. The trials are invariably paid for by the drug company whose drug is being tested, while the lead researcher is often recruited by the drug company, too.

It's an expensive business, of course.  It's reckoned that it costs the drug company between $25m and $30m (£13-15m) to support each drug with this level of support, but it's worth it, every drug company agrees.

So know you know why the doctor chooses the drug he does for you.  It may be right for you, but it's most definitely right for him.