Cranberry juice interferes with the absorption of a class of drugs called calcium channel blockers, as does grapefruit and grapefruit juice. This oral medication (eg. amlodipine) is widely prescribed to treat hypertension (sustained high blood pressure). Both cranberry and grapefruit juice also increase the blood-thinning effects of anticoagulants like warfarin and both juices inhibit the action of some statins.
I know of no interactions with metformin, As noted above, fruit juices in general, especially those made with tropical fruits like mango and pineapple, are best avoided altogether if you are type 2 diabetic. Anyone watching their waistline would be best advised to steer clear as well.
IMHO, drinks manufacturers should not be permitted to advertise fruit juice as counting towards your '5-a-day'. The mythical slogan reinforces the perception that their products are needed on health grounds. There are plenty of good sources of vitamin C and potassium without imbibing a sugary drink. Your average glass of unsweetened orange juice contains as much sugar as a similar serving of coke. On top of that, some brands are sweetened with added sugar!