I've never heard of toothpaste going bad. Yes, maybe if it falls into a box, is taken to the attic, and sets there for 10 years, probably then it wouldn't be any good. But just setting on the counter in the bathroom for a couple months? I've never heard of that.
But it happened. We had some Colgate toothpaste, two tubes that we were using. We basically got one real low, then I don't know how it happened -- we were on a trip or something and took the fuller one, then we kept using it, to the neglect of the other. The other set there no more than a couple months, around that.
The fuller one, as these things happen, became the less full one in time and that left the less full one to now be the fuller one. I started to use the new fuller one and guess what, the toothpaste had hardened to the point that I had to squeeze it with all my might, then just to get a very firm chunk of toothpaste that fell off the toothbrush. It landed with a plop. I tried another one and got the same thing. I tried to use the second piece and it had difficulty in there. Meaning I had to take drastic measures and throw the whole thing away.
This stuff was like the caulk that you use to seal up your bathtub. Although I'm thinking it would not have sustained itself if used like actual caulk.
C'mon, Colgate. What are you putting in your toothpaste. Another couple months and I would have had "Chalk In A Tube"