medicare when visiting US

I'm a US citizen, enrolled in Medicare A&B, living in London and visiting USA for 3 months. I'd be interested in other people's experiences: If you are enrolled in Medicare A&B, do you use it when you're visiting the US? what do you do if you need prescriptions. I can only find information for moving back to the US permanently (and signing up for Part D, etc) and that's not my case. I'll be returning to England

Part D has only been around since the Bush administration so I would assume that you would have to do what the over 65s used to have to do and pay full price.

While we were living in the USA our insurance prescription plan had a $500 prescription deductible so the first x months of every year would require us to pay full price for the prescriptions.