Most strep throat infections are mild and resolve completely with a standard course of antibiotics. But left untreated, strep can trigger complications that affect the heart, kidneys, and other organs, which is why it’s taken more seriously than a typical sore throat. Strep throat causes an estimated 5.2 million outpatient visits each year in the U.S. among people under 65, and the vast majority of those cases clear up without lasting problems.
What a Typical Case Looks Like
A straightforward strep infection means a sore throat, fever, swollen lymph nodes in the neck, and sometimes white patches on the tonsils. It can feel miserable, but the infection responds well to antibiotics. Penicillin or amoxicillin is the standard treatment, taken for 10 days. Within 24 to 48 hours of starting antibiotics, you’re no longer contagious, and most people feel noticeably better within two to three days.
Without antibiotics, strep throat usually resolves on its own within about a week. The reason doctors still recommend treatment isn’t primarily to speed recovery. It’s to prevent the rare but serious complications that can follow.
Rheumatic Fever and Heart Damage
The complication that makes untreated strep genuinely dangerous is rheumatic fever. This is an inflammatory reaction that develops when the immune system, while fighting the strep bacteria, mistakenly attacks the body’s own tissues. Fewer than 0.3% of people with strep throat develop rheumatic fever, so the individual risk is low. But when it does occur, the consequences can be severe: more than half of rheumatic fever cases lead to permanent scarring of the heart valves.
In the United States, rheumatic fever has become relatively uncommon thanks to widespread antibiotic use. Rates vary by region, from about 0.6 cases per 100,000 people in Tennessee to 3.7 per 100,000 in Utah. Globally, though, it remains a major cause of heart disease. The World Health Organization estimates 470,000 new cases of rheumatic fever and over 300,000 deaths from rheumatic heart disease occur each year, overwhelmingly in countries with limited access to antibiotics.
Kidney Inflammation After Strep
About 10 days after a strep throat infection, some people develop a condition called post-streptococcal glomerulonephritis, where the kidneys become inflamed. This happens when immune complexes formed during the infection deposit in the kidneys and trigger swelling. Roughly 470,000 cases occur worldwide each year.
The signs include dark or reddish-brown urine, swelling (especially around the eyes and in the legs), high blood pressure, fatigue, and reduced appetite. Unlike rheumatic fever, this complication can occur even if the strep infection was properly treated with antibiotics. The good news is that most cases, particularly in children, resolve on their own over weeks to months without permanent kidney damage.
Invasive Strep Infections
In rare cases, the same bacteria that cause strep throat can enter the bloodstream, muscles, or other deep tissues and cause life-threatening infections. The CDC estimates between 20,000 and 27,000 invasive group A strep cases occur in the U.S. each year, resulting in 1,800 to 2,400 deaths.
The two most dangerous forms are necrotizing fasciitis, a rapidly spreading infection of muscle and fat tissue, and streptococcal toxic shock syndrome, which causes dangerously low blood pressure and organ failure. About 20% of people with necrotizing fasciitis die from it, and the fatality rate for streptococcal toxic shock syndrome is around 60%. These conditions are medical emergencies with early warning signs that include fever, severe pain and swelling at a wound site, dizziness, confusion, rash, and abdominal pain.
These invasive infections are not a typical progression of strep throat. They usually occur when bacteria enter through a break in the skin or affect people with weakened immune systems. But they’re the reason public health agencies track group A strep closely.
Neuropsychiatric Effects in Children
A lesser-known complication is PANDAS, which stands for Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections. In some children before puberty, a strep infection appears to trigger a sudden, dramatic onset of obsessive-compulsive disorder, tics, or severely restricted eating. The leading theory is that the immune system, while attacking the bacteria, also attacks healthy brain tissue.
PANDAS is diagnosed when a child develops these psychiatric symptoms suddenly, within three months of a confirmed strep infection. It remains somewhat controversial in the medical community because it can be difficult to distinguish from other causes of childhood OCD or tic disorders, but the National Institute of Mental Health recognizes it as a distinct condition. Treatment focuses on clearing the underlying strep infection and managing the neuropsychiatric symptoms.
Signs That Need Prompt Attention
Most sore throats are caused by viruses, not strep, and don’t need antibiotics. But certain symptoms warrant a call to your doctor or a visit to urgent care:
- A sore throat with swollen, tender lymph nodes in the neck
- A sore throat lasting more than 48 hours
- Fever alongside a sore throat
- A rash accompanying the sore throat, which may indicate scarlet fever
- Difficulty breathing or swallowing
- No improvement after 48 hours on antibiotics, which could mean a resistant infection or a different diagnosis
Putting the Risk in Perspective
For any single episode of strep throat treated with antibiotics, the risk of serious complications is very low. The infection clears, you stop being contagious within a day or two, and there are no lasting effects. The danger lies almost entirely in untreated or undertreated infections, repeated infections, or the rare cases where bacteria become invasive.
Children between ages 5 and 15 get strep throat most often and are also the group most vulnerable to rheumatic fever and PANDAS. Adults get strep less frequently, but they’re not immune to complications. The single most effective thing you can do is finish the full 10-day course of antibiotics, even after you feel better. Stopping early increases the chance of the bacteria surviving and triggering an immune overreaction.