Not surprisingly given its genesis, consent of the individual involved is the uppermost principle
The voluntary consent of the human subject is absolutely essential. This means that the person involved should have legal capacity to give consent; should be so situated as to be able to exercise free power of choice, without the intervention of any element of force, fraud, deceit, duress, over-reaching, or other ulterior form of constraint or coercion; and should have sufficient knowledge and comprehension of the elements of the subject matter involved as to enable him to make an understanding and enlightened decision.
The Declaration of Helsinki, its first edition appearing in 1964, is in some sense an evolution of the Nuremburg Code. The Declaration more specifically deals with clinical research like drug trials. A key change is that the rule for consent has been wattered down, instead of consent being clearly expressed as 'absolutely essential', we have
In any research on human beings, each potential subject must be adequately informed of the aims, methods, sources of funding, any possible conflicts of interest, institutional affiliations of the researcher, the anticipated benefits and potential risks of the study and the discomfort it may entail. The subject should be informed of the right to abstain from participation in the study or to withdraw consent to participate at any time without reprisal. After ensuring that the subject has understood the information, the physician should then obtain the subject's freely-given informed consent, preferably in writing.
In a sense, this is useful clarification of the basic principle, i.e. calling out the different steps that warrant gathering consent. But, there are a number of SHOULDs in the above that might give those setting up clinical trials inappropriate leeway. At least, this is the argument made by Sonia Shah in her book 'The Body Hunters'. Shah lays out the current outsourcing model in which 'Big Pharma' conducts drug trials in third world countries, but brings any resulting benefits back to those first world customers who can pay.
Those running the trials argue that obtaining informed consent from some impoverished and illiterate non-English speaking subject is, at best, challenging and at worst, pretty much impossible. So, they argue, they shouldn't be held responsible if it turns out that trial participants turn out to not understand what they signed up for. Shah suggests that a simple solution to would be to quiz trial candidates for their understanding before they sign up. If they pass, they can participate. Otherwise, not.
This model might work in other consent contexts.