Can i take tetracycline & doxycycline at the same time? i am 40 all of a sudden broke out with cystic and whiteheads around mouth & chin. thanks!

Don't take both: As the name implies, these drugs are chemically very similar and they have exactly the same activity against various problems, including acne. There is never a reason to take both; it would just increase the chance of side effects (like stomach upset, sensitivity to sunburn) without improving your acne. Speak with the doctor(s) who prescribed them and/or taking care of your acne.