My understanding is that all ‘cyclines’ have a shelf life.
DOXYCYCLINE CAPSULES BP 100mg
Looks to be 4 years.
http://www.medicines.org.uk/emc/medicine/23950/spc
In some cases its been reported that its the binder that actually expires and not the medicine itself. Much of it depends on storage conditions but 2 years beyond the labeled shelf life for these medicines should be feasible.
I’ve read that the viability of tetracycline can be tested using titration but I don’t know how to do it.
I plan to rotate the older stuff and throw it away. Although I haven’t reached the first cycle yet.