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It's an antibiotic used to treat infection. Skin infections, sinus infections and sometimes acne (zits). It's can also be used to treat sexually transmitted diseases such as chlamydia.
Table 1. Therapeutic uses of doxycycline (Doryx, Vibramycin)
Traditional uses
Actinomycosis
Amebiasis
Anthrax
Balantidiasis
Brucellosis
Chancroid
Chlamydial infection
Cholera
Gonorrhea
Granuloma inguinale
Leptospirosis
Lymphogranuloma venereum
Melioidosis
Mycoplasma pneumoniae infection
Nocardiosis
Plague
Psittacosis
Q fever
Relapsing fevers
Rickettsialpox
Rocky Mountain spotted fever
Sexually transmitted disease
Shigella infections
Sinusitis
Syphilis
Trachoma
Traveler's diarrhea
Tropical sprue
Tularemia
Typhus (louse-borne, murine, and scrub)
Ureaplasma urealyticum infection
Yaws
Zoonotic infections
New or increased uses
Animal bites
Atypical pneumonias
Chlamydial infection
Ehrlichiosis
Enterococcus infections resistant to vancomycin (Lyphocin, Vancocin, Vancoled)
Falciparum malaria resistant to chloroquine HCl (Aralen HCl)
Human granulocytic and monocytic ehrlichiosis
Legionella infections
Lyme disease
Mycobacterium chelonei infection
Mycobacterium fortuitum infection
Penicillin-resistant Streptococcus pneumoniae
Toxoplasmosis