Here we have the same problems as we do with food. Any medical advice you read for travel warns against ice in drinks, fruit or vegetables that have been rinsed in water and drinking water that has not been boiled, filtered or chemically disinfected. Exactly how you are meant to eat a salad at a restaurant or drink a caipirinha on a hot day is not explained.
Yes, the tap water in Brazil is drinkable but in the big cities you really don’t want to. Not so much because of health concerns but because in the major cities the tap water has a lovely after taste. No one drinks tap water by choice and every home or office has a purified water dispenser. Bottled water is cheap and available everywhere.
Brushing your teeth with the tap water won’t kill you either but that refreshing minty taste will be replaced by a light chemical flavour. Come to think of it, these chemicals may do a better job of cleaning your teeth than you. Leave the toothpaste at home.
When buying cans of beer or soft drink it is traditional to give the top of the can a wipe first. This practice is more to do with psychology than any real defence against disease. Just wiping the rim of the can isn’t going to protect you from anything but it’s a habit most Brazilians practice and you’ll look like a local.
This habit comes from the commonly held view that the warehouses where pallets of stock are stored are not always the most hygienic of places. Thus the cans could be exposed to rats or vermin who do not have the modern flush toilets we take take for granted and instead are forced to use the rims of Coke Zero cans.
In reality your biggest health concerns when it comes to drinks will be hangovers. That and accidents caused from alcohol fuelled stupidity. Attempting to drink your way through all the different varieties of caipirinhas in a night can result in you cancelling all sightseeing activities for that day. It has been known for some graphic designers associated with this site to completely miss the Argentine side of Foz do Iguaçu due to the effects of alcohol.
It happens in many countries around the world and although I have not heard any first hand accounts of it in Brazil I'm sure it happens. Therefore the advice is the same abroad as it is at home. Only drink what you bought yourself and don't leave drinks lying around bars or clubs.