If I was being prescribed medicine by my doctor, I would expect them to tell me what the risks are. If I don't have the information, I can't give informed consent.
Have you ever received a prescription from your GP Jeremy?
In my experience the GP will tell you what the purpose of the prescription is in relation to the condition that you are suffering from, will check for any significant contraindications which might mean you couldn't take this particular medication and at best might mention the most significant side effects, particularly if they might require a bit of lifestyle change (e.g. don't drink, don't drive, don't operate machinery etc).
I have never know a GP to go through all the potential side effects and in most cases this would be so lengthy that you'd run out of consultation time.
In reality a GP is unlikely to know all the side effects of every drug (that isn't their job), so were you to ask him or her they would likely simply tell you that all the details are in the sheet in the box.
But again you are missing the point - a typical aspect of consent is that the person should have the opportunity to ask further questions (even if the answer will be 'read the leaflet'), but it doesn't negate valid consent if the person does not have further questions, nor if they chose not to acquaint themselves with all the details on the leaflet in the box - that is their choice. Provided the person has the ability to understand the information in the leaflet and has been provided with that information (which would be considered adequate and sufficient in legal terms) then the threshold for valid consent will be met presuming that the person makes the decision of their own free will, in other words voluntarily.