Immunotherapy works by releasing the brakes on your immune system so it can attack cancer cells, but that same unleashed immune response can turn against healthy tissue, particularly the lining of your gut. Diarrhea is the most common side effect of immune checkpoint inhibitors, affecting roughly 14% of people on PD-1 inhibitors and 35% of people on CTLA-4 inhibitors. Understanding why it happens, when to expect it, and how severe it can get helps you recognize the problem early.
How Immunotherapy Triggers Gut Inflammation
Immune checkpoint inhibitors don’t kill cancer cells directly. Instead, they block proteins that normally keep your immune system in check. One of these proteins, CTLA-4, acts as a brake during the early activation of T cells (the immune cells responsible for attacking threats). Another set of proteins, PD-1 and PD-L1, suppress T cells later in the process when they encounter tissue. Blocking either of these checkpoints ramps up the number and activity of T cells throughout your body.
The problem is that your gut lining is one of the most immunologically active places in your body. It constantly interacts with trillions of bacteria and needs a finely tuned immune balance to stay healthy. When checkpoint inhibitors flood the area with newly activated T cells, those cells can mistake the gut lining for a threat. They infiltrate the tissue, trigger inflammation, and damage the mucosal barrier that normally keeps intestinal contents from irritating the deeper layers of the bowel wall. The result is immune-mediated colitis: the colon becomes inflamed, absorbs less water, and produces the watery or frequent stools that patients experience as diarrhea.
This involves a complex interplay between T cells and the gut microbiome. Once T cells damage the intestinal lining, bacteria that normally stay on the surface can penetrate deeper into the tissue, which provokes even more immune activity. It becomes a cycle of damage and inflammation.
Why CTLA-4 Inhibitors Cause More Diarrhea
Not all immunotherapy drugs carry the same risk. CTLA-4 inhibitors cause diarrhea at roughly two and a half times the rate of PD-1 inhibitors. A meta-analysis of patients with advanced melanoma found all-grade diarrhea in 35.4% of those on CTLA-4 inhibitors compared to 13.7% on PD-1 inhibitors. The gap is even wider for severe cases: grade 3 or 4 diarrhea (meaning six or more loose stools per day, or symptoms that interfere with daily life) occurs in about 10% of patients on CTLA-4 therapy versus just 1 to 2% on PD-1 or PD-L1 agents.
The reason ties back to where each drug acts. CTLA-4 inhibitors intervene early in T-cell activation, producing a broader, less targeted immune response. This means more T cells are activated against a wider range of targets, including healthy gut tissue. PD-1 inhibitors act later and more selectively, which still carries risk but generates fewer off-target attacks on the bowel. Combination therapy, using both types together, pushes the risk even higher.
When Symptoms Typically Appear
Diarrhea from immunotherapy doesn’t usually start right away. Most cases emerge six to seven weeks after beginning treatment, with the median onset falling between two and five months. In melanoma trials using PD-1 inhibitors, the median time to onset was about eight weeks. For combination regimens or certain cancer types, it can take longer, with some studies reporting a median of 17 weeks or even five months.
Delayed onset is also possible. Around 3% of patients develop colitis more than 12 months after starting treatment. This means diarrhea can appear well after you’ve settled into a treatment routine, which is why staying alert to new digestive symptoms matters throughout the entire course of therapy and even after it ends.
Your Gut Bacteria Influence Your Risk
Researchers have found that the composition of your gut microbiome before treatment begins can predict whether you’ll develop diarrhea. Patients whose gut bacteria are dominated by certain species in the Firmicutes family tend to be more prone to colitis. One bacterium in particular, Gemmiger formicilis, has been identified as a marker of increased risk. Patients with severe diarrhea also tend to have higher levels of Stenotrophomonas and Streptococcus in their gut.
On the protective side, patients with higher levels of Bacteroides vulgatus and Faecalibacterium prausnitzii are less likely to develop colitis. Lower overall diversity in the microbiome is another warning sign. Patients with hepatobiliary cancers who developed severe diarrhea had notably less diverse gut bacterial communities. While microbiome testing isn’t yet a standard screening tool before immunotherapy, this research explains why two patients on the same drug can have very different digestive experiences.
What Happens Inside the Colon
When doctors investigate immunotherapy-related diarrhea with a colonoscopy, they typically see a recognizable pattern. The most common finding is a loss of the normal blood vessel pattern visible through the colon lining, present in 80% of cases. More than half of patients show mucosal bleeding, and about 69% have visible lesions, including superficial ulcers (22%) and deep ulcers (15%).
Under a microscope, tissue samples reveal inflammation of the crypts (the tiny glands in the colon wall) in 50% of cases, along with immune cell infiltration in 48%. Crypt abscesses, where immune cells collect inside the glands, appear in 37% of samples. Cell death in the lining is seen in about a quarter of cases. These findings help doctors distinguish immunotherapy-related colitis from other causes of diarrhea, such as infections, which is an important distinction because the treatments are very different.
How Severe It Can Get
Most immunotherapy-related diarrhea is mild to moderate, but it can escalate. Severe colitis, if left untreated, carries a risk of bowel perforation, estimated at 1.0 to 1.5% of all patients treated with checkpoint inhibitors. A perforation is a hole in the colon wall and is a life-threatening emergency requiring surgery. This is why oncology teams take new-onset diarrhea during immunotherapy seriously, even when symptoms seem manageable at first.
How It’s Treated
Treatment depends on severity, graded on a scale from 1 (mild increase in stool frequency) to 4 (life-threatening). For mild cases, your oncologist may simply monitor symptoms and recommend dietary adjustments while continuing immunotherapy.
For moderate cases (grade 2), the standard approach is high-dose steroids to suppress the immune attack on your gut. If symptoms don’t improve within 48 to 72 hours, the steroid dose is increased. When steroids alone aren’t enough, which can happen within two to five days, doctors add a biologic medication that targets inflammation more precisely. These biologics are given as infusions at specific intervals over several weeks.
For severe or life-threatening cases (grades 3 and 4), treatment starts with intravenous steroids, and biologic therapy is added quickly if there’s no improvement within a few days or if colonoscopy reveals ulcers. Immunotherapy is typically paused during treatment and may be permanently discontinued depending on the severity. Most patients with moderate colitis recover within weeks once appropriate treatment starts, though steroid tapers are gradual to prevent relapse.
Other Causes to Rule Out
Not every case of diarrhea during immunotherapy is caused by the immune system attacking the colon. Infections, particularly Clostridioides difficile (a bacterial infection common in hospitalized patients), can mimic immune-mediated colitis and require completely different treatment. Oncology teams routinely test stool samples for infections before starting steroids, since suppressing the immune system during an active infection would make things worse. Other possibilities include side effects from concurrent medications, dietary changes, or cancer-related factors, all of which need to be considered before attributing diarrhea to the immunotherapy itself.