https://www.drugs.com/answers/long-doxycycline-safe-979226.html
"This group of antibiotics called tetracyclines can actually become toxic with age.
They are no longer any good-get rid of them!
With other classes of antibiotics, mainly age effects their potency by decreasing it so they are no longer effective but tetracyclines can actually harm you to take them because the chemical composition can become toxic."