Doxycycline treats a wide range of bacterial infections, from common conditions like chlamydia and acne to serious illnesses like Rocky Mountain spotted fever and pneumonia. It belongs to the tetracycline class of antibiotics and works by stopping bacteria from making the proteins they need to grow. Few antibiotics cover as many different types of infections, which is why doxycycline remains one of the most frequently prescribed medications worldwide.
Sexually Transmitted Infections
Chlamydia is one of the most common reasons doctors prescribe doxycycline. The standard treatment is a 7-day course, taken twice daily, and it’s now considered the preferred first-line option over the single-dose azithromycin that was previously favored. Doxycycline effectively clears chlamydia infections in the urethra, cervix, and rectum.
Beyond treating active infections, doxycycline has a newer role in STI prevention. A single 200 mg dose taken within 24 hours of condomless sex (and up to 72 hours after) can reduce the risk of contracting chlamydia and syphilis. This approach, called doxy-PEP, is primarily recommended for men who have sex with men and transgender women at higher risk for STIs. It’s less effective against gonorrhea, partly because resistance to tetracycline antibiotics has been climbing in the bacteria that cause it.
Tick-Borne Diseases
Doxycycline is the go-to treatment for nearly every major tick-borne illness in the United States. Rocky Mountain spotted fever, which can become life-threatening within days if untreated, requires prompt doxycycline therapy. The CDC recommends continuing treatment for at least 3 days after fever breaks, with a minimum course of 5 to 7 days. This applies to adults and children of all ages, including young children, where the urgency of treatment outweighs any concerns about tooth staining.
Lyme disease, anaplasmosis, and ehrlichiosis all respond to doxycycline as well. For early Lyme disease, a typical course runs 10 to 21 days. Because tick bites often carry uncertainty about which pathogen might be involved, doxycycline’s broad coverage against multiple tick-borne organisms makes it especially practical. If you’ve been bitten by a tick in an area where these diseases are common, doxycycline is almost always what you’ll be prescribed.
Respiratory Infections
Doxycycline is an effective option for community-acquired pneumonia, particularly milder cases that can be treated at home. A meta-analysis of clinical trials found that about 87% of patients with mild-to-moderate pneumonia were cured with doxycycline, a rate comparable to or slightly better than alternative antibiotics tested in the same studies.
It works especially well against several bacteria that commonly cause respiratory infections, including the organisms behind “walking pneumonia” and certain strains of bacteria that cause bronchitis and sinus infections. However, resistance among pneumococcus (the most common cause of bacterial pneumonia) is a concern, so doctors may choose a different antibiotic if that particular bacterium is suspected. Doxycycline also covers infections caused by certain strains of Haemophilus influenzae, a frequent culprit in flare-ups of chronic bronchitis.
Skin Conditions: Acne and Rosacea
Doxycycline is FDA-approved as an add-on treatment for severe acne. At standard doses, it reduces the inflammatory bacteria on skin, but it also has anti-inflammatory properties that help independently of its germ-killing ability. Treatment courses for acne typically run several months. One study found that a low dose taken twice daily for 6 months significantly reduced acne symptoms without disrupting the skin’s normal bacterial balance.
For rosacea, doxycycline is often prescribed at lower, “sub-antimicrobial” doses, meaning the dose is too low to kill bacteria but still reduces the inflammation driving redness and bumps. A specialized 40 mg once-daily formulation is FDA-approved specifically for rosacea. Treatment courses generally last 8 to 16 weeks, with doses ranging from 40 mg to 100 mg daily depending on severity. Because lower doses don’t act as true antibiotics, they carry less risk of contributing to antibiotic resistance.
Malaria Prevention for Travelers
Doxycycline doesn’t treat malaria once you have it, but it’s one of the most commonly used medications to prevent it while traveling to high-risk areas. You take 100 mg once daily, starting 1 to 2 days before entering a malaria zone, continuing every day while there, and then for 4 full weeks after leaving. That 28-day tail is important because malaria parasites can linger in the body after you’ve left the area.
The short lead time makes doxycycline a good option for last-minute travelers, since some alternatives require starting weeks in advance. The tradeoff is that you need to take it every single day without missing doses, and the extended post-travel course can be easy to forget.
Less Common but Serious Infections
Doxycycline is approved for several rarer but potentially dangerous infections. Cholera, plague, tularemia (rabbit fever), and anthrax all fall within its spectrum. It’s also used for brucellosis (typically combined with another antibiotic) and Q fever, a disease spread by livestock. These conditions are uncommon in everyday life but important in specific occupational, geographic, or bioterrorism contexts.
Bartonellosis, sometimes acquired through sandfly bites in South America, and granuloma inguinale, a rare STI, are also on the FDA-approved list. For several of these less common infections, doxycycline may be one of very few effective treatment options.
What Doxycycline Won’t Treat
Doxycycline is purely antibacterial and has no effect on viral infections like the common cold, flu, or COVID-19. It also doesn’t treat fungal infections. While it’s useful for some STIs, it’s not the right choice for gonorrhea treatment due to high rates of resistance, and it’s not a primary treatment for syphilis (penicillin remains the standard for that).
Resistance is an evolving concern more broadly. Recent surveillance data shows that tetracycline resistance among gonorrhea-causing bacteria has been increasing across all population groups, driven partly by the growing use of doxycycline for STI prevention. For any infection where resistance is possible, your doctor may order a culture to confirm doxycycline will actually work before committing to a full course.
Side Effects and How to Avoid Them
The most distinctive side effect of doxycycline is increased sun sensitivity. Your skin burns more easily and more severely while you’re on the medication, so sun protection matters more than usual during treatment.
Esophageal irritation is the other well-known risk. Doxycycline can cause ulcers in the esophagus if a pill gets stuck or dissolves before reaching the stomach. This can happen even after a single dose and causes sudden chest pain, difficulty swallowing, and heartburn. Prevention is straightforward: take the pill with a full glass of water, stay upright for at least 30 minutes afterward, and never take it right before lying down or going to sleep.
Dairy products, calcium supplements, and vitamins containing zinc or magnesium can interfere with absorption if taken too close together. Keep a two-hour gap between doxycycline and any of these. Nausea is common, and taking the medication with food (just not dairy) helps reduce stomach upset.
Use During Pregnancy and in Children
Tetracycline antibiotics as a class are known to stain developing teeth, which has historically limited doxycycline use in pregnant women and young children. However, the actual risk with doxycycline specifically is less clear-cut than once believed. The FDA notes that no published human data confirms doxycycline causes tooth staining in fetuses, though it can’t be ruled out due to the class effect. A large case-control study found only a weak association between doxycycline use during pregnancy and birth defects.
For children, guidelines have shifted in recent years. Doxycycline is now recommended for children of all ages when treating tick-borne diseases like Rocky Mountain spotted fever, where the danger of the infection far exceeds any cosmetic risk to teeth. Short courses appear to carry minimal staining risk. Doxycycline does pass into breast milk, so prolonged use during breastfeeding warrants a conversation about alternatives.