When doing a clinical trial in the uk, can doctors/pharmacists detect a placebo?

when doing a double blind trial (this means neither docror/pharmacist nor patient knows if a placebo or trial medecine is been taken), does the pharmacist have a criteria for determining if a placebo was taken during the trial? i mean what is to stop someone from doing a trial and not taking the trial drug/placebo?

Well, all doctors participating on the study should know the drugs that are being used, (comparing two drugs, comparing to placebo), but the whole point of a double blind is that neither the doctor nor the patient know which one is he taking (otherwise, it wouldn't be a double blind, but a single blind)

I didn't get that last question, what do u mean stopping someone?, of course patients have the right to withdraw from the study at anytime (poor investigator)

When the patient agrees to take part in the study, they are randomised so that they may get either the drug or placebo.

In a double-blinded study, the doctor / nurse / treating healthcare professional will be given the treatment (let's say it comes in a bottle) in a numbered bottle. This bottle, and the number on it, will mean nothing to the patient or doctor/nurse. There will somewhere be a database with numbers lined up alongside specific doses of the treatment or the placebo.
Patients and healthcare staff should not be made aware of whether they got the placebo or the treatment.

But naturally the patient will try and guess what they got! When I took part in a study, I reckon I got a placebo (it was a study looking at pain relief, and I felt no relief, so I hope it was the placebo!).

As far as I know, a nurse will either administer the drug or watch you take it.

from my experience you take the drug in front of a nurse