Hi
That's a very good point. There are basically two types of medical trials - ones for 'healthy volunteers' and ones with a certain condition which the trial is trying to find a solution for. Most tend to be for healthy volunteers. If you are healthy then they are quite easy to get onto. When I say 'healthy' I mean you can't be taking any regular medication, have no previous medical problems and, most of the time, be a non smoker. You even have to have a certain blood pressure.
You go on a 'screening' first to test you for alcohol, weight, blood pressure, blood test etc and if you pass all that they will contact your GP. If there have been any problems in the past you are immediately barred.
Someone like you could probably not do a medical trial because of your low white blood count and allergy. However, if they were testing a new allergy drug then you would obviously be considered.
Someone I knew fitted all the criteria for being 'healthy' - he passed all the tests but on the screening day he had slightly high blood pressure and was refused (and blood pressure can change even during the day). I think many people are economical with the truth when it comes to drinking however. You are not supposed to drink more than the daily guidance amount and when asked, everybody always says ' "well, I never drink during the week and I might perhaps have a couple of pints during the weekend' hmmm (us Brits are not that sensible!)....
So basically to your question, yes - there are really strict when it comes to the screening. Because of the money is so attractive to people they must get people lying all the time but there is no point because the medical records show everything.
All the best!