Hello, Angie!

Doxytet (Doxycycline) does not become dangerous, once expired, but it can be less effective, which could result in your developing antibiotic resistant bacteria. If the option is available, it would really be best to dispose of it and obtain a fresh supply.

The U.S. FDA lists its typical side effects as possibly including nausea, dizziness, headache, non-allergic skin rash and diarrhea.

• According to the following article from Jase Medical journal, Doxycycline does not degrade into any toxic components overtime:
  When does Doxycycline become toxic?

• The FDA even went so far as to extend the expiration dates of certain lots doxycycline, as referenced in the following link:
  Expiration date extensions of certain lots of doxycycline hyclate capsules

Additional information about this medication can also be found on MedlinePlus.

Hope this helps!