Doxycycline hyclate is a widely prescribed antibiotic in the tetracycline class, used to treat a broad range of bacterial infections affecting the skin, lungs, urinary tract, and reproductive system. It also plays a role in preventing certain diseases, including malaria and Lyme disease. Few antibiotics cover as many conditions, which is why doxycycline is one of the most commonly dispensed medications in the United States.
Sexually Transmitted Infections
One of the most common reasons people are prescribed doxycycline hyclate is for sexually transmitted infections. It is the CDC’s first-line recommended treatment for chlamydia in adults and adolescents, typically taken as 100 mg twice a day for seven days. It also treats several other STIs, including nongonococcal urethritis, lymphogranuloma venereum, granuloma inguinale, uncomplicated gonorrhea, and chancroid.
More recently, the CDC issued guidelines in 2024 for using doxycycline as post-exposure prophylaxis after potential STI exposure, a strategy sometimes called “doxy-PEP.” This involves taking doxycycline after a high-risk sexual encounter to reduce the chance of developing bacterial STIs like chlamydia and syphilis.
Respiratory Tract Infections
Doxycycline hyclate treats several types of respiratory infections. These include pneumonia caused by Mycoplasma (sometimes called “walking pneumonia”), upper respiratory infections from common bacteria like Streptococcus pneumoniae and Haemophilus influenzae, and psittacosis, a lung infection spread by birds. It is often chosen when a patient has a penicillin allergy or when the specific bacteria involved are known to respond to tetracycline-class drugs.
Lyme Disease Prevention
If you’ve been bitten by a tick in an area where Lyme disease is common, a single 200-mg dose of doxycycline can reduce your risk of developing the illness. This preventive dose works best when taken within 72 hours of removing the tick. Not every tick bite warrants it. The CDC recommends considering prophylaxis when the tick is an Ixodes (blacklegged) tick and appears engorged with blood, which suggests it was attached long enough to transmit the Lyme-causing bacteria. A flat, unfed tick is unlikely to have passed along the infection.
Acne and Rosacea
Doxycycline hyclate is frequently prescribed for moderate to severe acne and for the inflammatory bumps and redness of rosacea. For rosacea, doses typically range from 40 mg to 100 mg once or twice daily, with treatment lasting six to 12 weeks. The lower 40-mg dose is considered “sub-antimicrobial,” meaning it works primarily as an anti-inflammatory rather than killing bacteria. This approach reduces inflammatory lesions while lowering the risk of contributing to antibiotic resistance. Expect three to four weeks of treatment before you see substantial improvement.
Malaria Prevention for Travelers
Doxycycline hyclate is one of several options for preventing malaria during travel to regions where the disease is transmitted. You start taking one dose per day a day or two before entering a malaria zone, continue daily while there, and keep taking it for 28 consecutive days after leaving. That extended tail is important because malaria parasites can linger in the body, and stopping the medication too soon leaves you vulnerable.
Anthrax Exposure
Following suspected or confirmed exposure to anthrax spores, doxycycline hyclate is a key part of post-exposure treatment. Adults and children weighing 76 pounds or more take 100 mg twice daily, 12 hours apart. Most people receive a 10-day supply initially, but the full course can extend to 60 days depending on the level of exposure. This prolonged course is necessary because anthrax spores can remain dormant in the body for weeks before causing illness.
Other Bacterial and Rare Infections
Doxycycline hyclate covers a surprisingly long list of less common but serious infections:
- Rocky Mountain spotted fever and other rickettsial diseases: tick-borne illnesses that can become life-threatening without prompt treatment.
- Plague: caused by Yersinia pestis, still seen in parts of the western United States.
- Cholera: a severe diarrheal illness caused by Vibrio cholerae.
- Tularemia: sometimes called “rabbit fever,” transmitted through insect bites or contact with infected animals.
- Brucellosis: typically contracted from unpasteurized dairy products or infected livestock, treated with doxycycline in combination with another antibiotic.
- Urinary tract infections: caused by certain gram-negative bacteria when testing confirms they are susceptible to the drug.
It is also approved for two eye infections: trachoma and inclusion conjunctivitis, both caused by Chlamydia trachomatis.
Hyclate vs. Monohydrate
Doxycycline comes in two salt forms: hyclate and monohydrate. They deliver the same active drug and treat the same conditions. The monohydrate form is less water-soluble, and there is some suggestion it causes fewer stomach-related side effects, though published evidence showing a clear difference in patient experience is limited. If your pharmacy substitutes one for the other, the therapeutic effect is the same.
Side Effects and Practical Tips
The most common side effects of doxycycline hyclate are nausea, vomiting, diarrhea, and esophageal irritation. That last one is preventable: take the pill with a full glass of water and stay upright for at least an hour afterward. Lying down too soon after a dose can cause the tablet to lodge in your esophagus and irritate the lining, leading to a painful burning sensation in your chest. Taking it on a full stomach also helps reduce nausea.
Photosensitivity is another well-known issue. Your skin becomes more prone to sunburn while taking doxycycline, so wearing sunscreen and limiting direct sun exposure is important during treatment. This effect can be surprisingly intense, even on overcast days or with brief sun exposure that normally wouldn’t cause a burn.
Doxycycline can bind to calcium, iron, and similar minerals, which reduces how much of the drug your body absorbs. If you take calcium supplements, iron pills, or antacids, separate them from your doxycycline dose by at least two hours. Dairy products contain calcium but are less of a concern when doxycycline is taken with a meal, since the food helps buffer the interaction.