Seeing this leads me once again to one of my pet subjects - leaflets that come with all medicines. These leaflets cover a number of key points including purpose i.e. what the medication is for, who it for, who should not take it, how much to take, when to take and potential side effects etc.
Medicines work in different ways and so when to take them will vary. Some are taken on an empty stomach, others are taken with a meal, then others are taken after a meal, some before a meal and some at night time etc.
The leaflet will tell you how the manufacturer recommends you take this medication, but the GP might give you different instructions which then pharmacist should then advise the patient by printing this on the label.
One other thing I should mention is the interaction medicines have with other medicines and sometimes foods. So there will be information on what foods not to take the medication with and what conflicts there may be with other medications - this may refer you back to the GP, though of course had the GP been thorough they would have known this when they prescribed the medication.
Taking Medication - Read Manufacturer's Leaflet
Just some thoughts I have which may or may not help you.