Doxycycline is contraindicated in pregnancy because it can cross the placenta and affect fetal bone and teeth development, leading to permanent discoloration of teeth and potential inhibition of bone growth. Additionally, it may cause teratogenic effects and should be avoided to prevent harm to the fetus. While doxycycline can be used cautiously in liver disease and renal failure, its risks in pregnancy outweigh benefits, making it a clear contraindication. Hypertension is not a concern for doxycycline use.

Reference: Goodman & Gilman's The Pharmacological Basis of Therapeutics, Brunton et al., 13th Edition.