Driving me crazy for years. No one knows. It's a movie from the 80s where a man breaks into a mans house and puts a deadly chemical in toothpaste which makes the man using it explode. I remember cows and an entrance to an underground facility. Please help!!

I actually did not remember this movie, having read your description numerous times. Suddenly, my thoughts brought me to a very vague memory. I was astonished to find the film, one I haven't seen since about 1983!

Endangered Species (1982)
Endangered Species (1982) - IMDb

From IMDb synopsis: I recall seeing "Endangered Species" on a late night movie channel when I was very young. Actually, it seemed to come on late night movie channels quite frequently from about 1985-1987. I remembered the film being mysterious, dark, and a little bit unsettling. I remembered strange parts of the film, especially the scene where germ warfare is put on a man's toothbrush, and the gruesome effects afterwards. (my bold words)