Why Keytruda (pembrolizumab) can make it hard to keep weight on
Some people lose weight during treatment with Keytruda. That can happen for several reasons, including reduced appetite, nausea or diarrhea, and fatigue. Immune-related side effects can also affect the gut, liver, thyroid, or other organs that play a role in digestion and metabolism, which can then make it harder to maintain weight.
If weight loss is happening, it’s important to treat it as a medical issue rather than only a “side effect,” because some causes are reversible when caught early (for example, thyroid problems or inflammation in the GI tract).
What other symptoms should raise concern (tell your oncology team urgently)
Contact your oncology team promptly if weight loss is paired with any of the following:
- Persistent diarrhea or abdominal pain
- Vomiting or inability to eat
- Ongoing nausea
- Yellowing of the eyes/skin, dark urine, or severe right-sided abdominal pain
- Severe fatigue, cold intolerance, constipation, or feeling unusually weak (possible thyroid or metabolic issues)
- Worsening shortness of breath or cough (systemic effects)
- Blood in stool or black/tarry stools
These can point to immune-related inflammation that may require steroids or other treatment and sometimes a temporary pause of immunotherapy, depending on severity.
What your doctor can check to find the reason for weight loss
Your team may evaluate common, treatable causes such as:
- Bloodwork for dehydration, anemia, inflammation, liver issues, kidney function, and electrolytes
- Thyroid testing (TSH and free T4) because thyroid dysfunction can affect appetite, energy, and weight
- Stool testing and/or exam if diarrhea is present
- Liver tests and symptom review if jaundice or abdominal symptoms occur
- Other assessments based on your specific symptoms and cancer type
If immune-related toxicity is suspected, the plan typically depends on which organ system is involved and how severe the symptoms are.
Practical steps patients often use to maintain or regain weight during Keytruda
These are general strategies you can discuss with your team or a dietitian:
- Eat smaller, more frequent meals instead of relying on large portions.
- Choose calorie-dense foods (for example, adding healthy fats like olive oil, nut butters, avocado, or full-fat dairy if tolerated).
- Use nutrition drinks or shakes if solid foods are hard to manage.
- Treat nausea/diarrhea promptly with the meds your oncology team recommends, because waiting can worsen weight loss.
- Keep a simple log of intake and symptoms so you can report trends early (what you can eat, nausea level, stool frequency, etc.).
A dietitian familiar with oncology can tailor an approach to your appetite, digestion, and lab results.
When to consider holding treatment or changing the plan
Weight loss alone doesn’t automatically mean Keytruda must stop, but significant or rapid loss, inability to keep food down, or immune-related toxicity that is moderate to severe may change the treatment approach. Your oncology team will weigh:
- Severity of symptoms
- Any immune-related diagnosis
- Lab abnormalities
- How well you’re tolerating treatment
If immune-related colitis, hepatitis, endocrinopathies, or other toxicity is identified, treatment usually targets the underlying cause.
If the issue is diarrhea or stomach inflammation
If you’re losing weight because of GI symptoms, the most important step is to report it quickly so clinicians can rule out immune-mediated colitis. Early management can prevent dehydration and further weight loss.
Are there alternatives if weight loss is severe or persistent?
Options depend on your cancer type and what’s causing the weight loss. Sometimes the answer is supportive care and treating the side effect; other times it can involve changing timing, using additional meds for symptoms, or changing cancer therapy. Your oncology team can review whether continued Keytruda is appropriate for your situation.
What to do next (so your team can act fast)
Reply with:
1) Your cancer type and how long you’ve been on Keytruda
2) How much weight you’ve lost and over how many weeks
3) Whether you have diarrhea, nausea/vomiting, constipation, belly pain, or trouble eating
4) Any recent lab abnormalities your team mentioned (if you know them)
With that, I can help you map likely causes and what to ask your oncology team to check first.