Drinking the Tap Water

Drinking water quality in the EU is regulated by the Drinking Water Directive. Tap water should be safe to drink everywhere in the EU. https://en.wikipedia.org/wiki/Drinking_Water_Directive_2020#
There only problem I can see is old buildings with old pipes that may be source of contamination

We lived in Germany for four years and drank the tap water. No problem there. Most of Europe should be fine, however not Russia.

If you visit third world countries, DO NOT drink the tap water. Places like Egypt, Morocco, Kenya, Peru, Ecuador, etc.

I've heard that about Lithuania, and I did some digging. Most of the warnings come for 10 year old sources. Everything in the last 5 years says it's okay. But if you are concerned, that's fair, drink bottled water.

In the EU, while the water that is distributed may be safe, a lot of those old buildings have questionable conditions that would never meet codes developed to ensure the water remains safe in the EU or the US. Quite often upgrades are not required until major renovations are done.

I have no sources to cite. I didn't encounter any warnings prior to my trip to the Baltic countries last year.

I spent 16 days in Latvia (mostly Riga but also Liepaja) and 18 days in Lithuania (Vilnius, Klaipeda and Kaunas). My beverage of choice is water, though I may occasionally have bought a Coke. The only time I purchased bottled water was in restaurants where I decided not to request tap water for one reason or another. That was probably once a day on average.

August 2023: I used tap water to brush my teeth in Latvia, Lithuania and Estonia. I drank water and drinks with ice (likely tap water) during my stay in all 3 countries. No ill effects.

I was in both countries in October 2022 and had no issues with drinking tap water (none in Estonia either).

I'm drinking water now and....ghhhaaarrrgg...

Thump...

Spent three weeks in Latvia, Lithuania and Estonia during 2018. In the big cities, small towns, and some pretty remote areas.

While I don't often drink "tap water" in places where I have concerns about water quality, I am generally very careful to not allow a single drop in my mouth, nose or other orifices - so in those places, I always brush my teeth with bottled water and take careful steps to keep the odd drop of water out while washing, showering, etc. On other trips I have gotten sick pretty easily from less-than-perfect water & questionable sanitation overseas, so I tend to be pretty obsessive, bordering on complete paranoia, about "water discipline" eschewing ice in drinks, regarding uncooked produce with suspicion, etc.

In the Baltic countries, I took no such measures. I didn't drink a lot of tapwater, but I brushed my teeth with it every day, enjoyed ice in drinks, crispy salads, rinsed off fresh produce in sinks (wonderful berries in mid-summer!) - all things I would never dream of doing in Mexico or most of the developing world.

I had no stomach issues at all in the Baltics.

Unless you were staying/eating in a place that looked visibly funky, I would not be terribly concerned. Maybe skip the water fountains in the Museum of The Occupation and other old Soviet gulag charmers...

I'm not a good one to ask, as I drank tap water throughout Quintana Roo, Mexico without any issues. But Brita filters will take care of amoebas, bacteria and other nasty stuff, so I think you'll be good.

Brita sports bottle do not filter out bacteria. They are different from the home Brita filters.