Untreated chlamydia can cause serious damage to the reproductive system in both men and women, lead to complications during pregnancy, trigger joint inflammation, and affect newborns. With over 1.5 million cases reported in the U.S. in 2024 alone, it remains the most commonly reported bacterial sexually transmitted infection, and many of its worst consequences develop silently in people who never had symptoms.
Pelvic Inflammatory Disease in Women
The most significant risk for women is pelvic inflammatory disease, or PID. Mathematical modeling estimates that about 22% of women with untreated chlamydia will eventually develop PID, an infection that spreads from the cervix into the uterus, fallopian tubes, and surrounding tissue. PID can cause chronic pelvic pain, but the damage it does inside the body is often more consequential than the symptoms it produces.
The core problem is scarring. When chlamydia infects the cells lining the fallopian tubes, it triggers an inflammatory response that converts normal tissue into scar-producing cells. These cells deposit excess collagen and other structural proteins, gradually thickening and distorting the tubes. Over time, this can partially or fully block them. The result is tubal factor infertility, where eggs can no longer travel from the ovaries to the uterus. If a fertilized egg implants in a damaged tube instead of the uterus, the result is an ectopic pregnancy, a potentially life-threatening emergency.
What makes this especially dangerous is that PID itself can be subtle. Some women experience lower abdominal pain, unusual discharge, or pain during sex. Others feel nothing at all and only discover the damage years later when they have difficulty getting pregnant.
Effects on Male Fertility
In men, chlamydia most commonly causes inflammation of the tube that carries sperm from the testicle, a condition called epididymitis. Chlamydia is responsible for 40 to 80% of epididymitis cases. Symptoms typically include pain and swelling on one side of the scrotum, sometimes with fever. Left untreated, the infection can spread to the testicle itself or the prostate.
The fertility effects go beyond blocked tubes. Chlamydia directly damages sperm cells, reducing their ability to move, increasing the number of non-viable sperm, and triggering DNA fragmentation within sperm. Men with chlamydia can see changes in sperm concentration, motility, and shape. The encouraging news is that these effects appear to be at least partially reversible: one study of infertile men who tested positive found that after a course of antibiotics, sperm quality improved by more than 57% across most measures within 30 days.
Risks During Pregnancy and for Newborns
Chlamydia during pregnancy raises the stakes considerably. Infection has been linked to miscarriage, premature rupture of membranes, preterm labor, and low birth weight. Some research suggests untreated chlamydia may increase the risk of preterm delivery by two to four times.
The infection can also pass directly to the baby during vaginal delivery. Without treatment, vertical transmission rates are estimated at 50 to 70%. Among exposed newborns, 30 to 50% develop conjunctivitis, an eye infection that appears in the first few weeks of life. Another 10 to 20% develop pneumonia. Treatment for these neonatal infections is only about 80% effective on the first course, so some infants need a second round.
Reactive Arthritis
Chlamydia can trigger joint inflammation even after the genital infection clears, a condition called reactive arthritis. The bacteria travel from the urogenital tract to the joints inside circulating immune cells called monocytes. Once there, the organism persists inside those cells and provokes an ongoing inflammatory response.
Reactive arthritis typically affects the knees, ankles, and feet, often alongside eye inflammation and urinary symptoms. Some people recover within months, but others develop a chronic form. Genetics play a role: people who carry a specific immune system gene (HLA-B27) are more likely to develop reactive arthritis and to have it become long-lasting, because their immune system is less effective at clearing the bacterial remnants that drive inflammation.
Rectal and Throat Infections
Chlamydia doesn’t only infect the genitals. Rectal chlamydia, typically acquired through anal sex, is common and usually caused by the same strains that cause genital infection (serovars D through K). About two-thirds of rectal infections produce no symptoms at all. When symptoms do appear, they tend to be mild: anal itching, a mucous discharge, or mild discomfort.
A more aggressive form of rectal chlamydia is caused by different strains known as LGV (lymphogranuloma venereum). This version causes severe rectal pain, bloody or pus-filled discharge, constipation, and fever. If untreated, LGV can progress to a chronic stage that destroys tissue, forming strictures and fistulas in the rectum that can mimic Crohn’s disease. In the most severe cases, lymph node destruction leads to permanent swelling of the genital tissue. LGV can also cause painful, swollen lymph nodes in the groin, typically on one side, sometimes progressing to large abscesses that may need to be drained.
Pharyngeal (throat) chlamydia is generally considered asymptomatic and is detected primarily through screening rather than because of complaints.
How Treatment Works
Chlamydia is curable with antibiotics, and early treatment prevents nearly all of the complications described above. The CDC’s current first-line recommendation is a seven-day course of an oral antibiotic taken twice daily. A single-dose alternative exists but has fallen to second-line status because it is less effective, particularly for rectal infections. One randomized trial found 100% cure rates with the seven-day regimen for rectal chlamydia, compared to only 74% for the single-dose option.
The challenge is timing. Because chlamydia is asymptomatic in roughly 70% of women and up to 50% of men, many people carry the infection long enough for damage to begin before they ever get tested. Scarring in the fallopian tubes, changes to sperm quality, and joint inflammation can all develop during months or years of silent infection. Regular screening, particularly for sexually active women under 25, is the main tool for catching infections before they cause lasting harm.