Chlamydia is a sexually transmitted infection (STI) caused by Chlamydia trachomatis bacteria. It is often asymptomatic, so regular screening is recommended for people at risk. Chlamydia can be easily treated with antibiotics, but if left untreated, it can lead to serious complications.

Chlamydia is the most common bacterial STI in the United States, and new cases have risen steeply over the past three decades. More than 1,640,000 cases were reported in 2021, but many infections remain undiagnosed. Around two thirds of chlamydia cases occur among young people ages 15 to 24. Young women and gay and bisexual men both have high rates. HIV-positive people with chlamydia are more likely to transmit the virus, and HIV-negative people with chlamydia are more likely to acquire HIV.

A majority of people with chlamydia have few or no symptoms, but some develop inflammation of the cervix, urethra or rectum. Depending on the site of infection, symptoms may include pain or burning during urination, white or grayish discharge from the penis or vagina, rectal discharge and pain or a sore throat. Less commonly, chlamydia can affect the eyes, causing conjunctivitis.

If left untreated, chlamydia can lead to long-term complications. Women can develop pelvic inflammatory disease, or infection of the reproductive organs. Symptoms may include fever, pain in the lower abdomen, pain or bleeding during intercourse and vaginal bleeding between menstrual periods. Over time, scar tissue can form around the uterus, fallopian tubes and ovaries, which may lead to chronic pelvic pain, infertility and ectopic pregnancy. Complications in men can include inflammation of the testicles or prostate.

Prevention and Treatment

Chlamydia spreads primarily through vaginal, anal and oral sex. Pregnant people with chlamydia can transmit the bacteria to their babies during delivery. Chlamydia doesn’t spread through casual contact, such as hugging or kissing. Infection does not confer lasting immunity, so it is possible to get chlamydia multiple times.

Condoms and dental dams offer good protection against chlamydia. Taking the antibiotic doxycycline as post-exposure prophylaxis after sex—known as doxyPEP—reduces the risk of acquiring chlamydia, gonorrhea and syphilis for gay men and transgender women, but this approach didn’t work for cisgender women in one study (see “DoxyPEP 101”). There is currently no vaccine for chlamydia, but research is underway.

Get tested if you have STI symptoms or if a sex partner tests positive. Chlamydia is usually diagnosed with a urine test or using a swab to take a fluid sample from the vagina, penis, rectum or throat. The Centers for Disease Control and Prevention recommends annual chlamydia screening—even if asymptomatic—for sexually active women under age 25 and older women at increased risk. Men who have sex with men should be screened at least annually or every three to six months if they’re at increased risk—for example, if they have multiple sex partners, are HIV positive or are taking pre-exposure prophylaxis (PrEP).

If diagnosed with chlamydia, inform sex partners and hold off on having sex until treatment is completed. Chlamydia can be treated and cured with antibiotics. The usual treatment involves either a seven-day course of doxycycline pills or a single dose of oral azithromycin. These drugs are safe and generally well tolerated. Even if symptoms improve, it’s important to take the full course of medication to prevent drug resistance.

People with STIs often experience shame or stigma, which can discourage seeking care. While chlamydia can be cured, antibiotics do not reverse existing damage, so it’s important to get tested and treated promptly.