Can Keytruda (pembrolizumab) cause weight gain?
Weight changes can happen during treatment with Keytruda, but weight gain is not usually one of the most common expectations. People may gain weight for several reasons during immunotherapy, including changes in appetite, activity level, or the underlying cancer and overall health. That said, Keytruda can also cause immune-related side effects that affect hormone function and metabolism, which can lead to weight gain in some patients.
What immune side effects could lead to weight gain?
Keytruda can trigger immune-related conditions, including thyroid disorders. If hypothyroidism develops (low thyroid hormone), it can cause symptoms such as fatigue and weight gain. Other immune-related endocrine problems can also affect weight, though the thyroid link is the best-known mechanism for weight gain during immune checkpoint inhibitor therapy.
What about opposite effects—can Keytruda cause weight loss instead?
Yes. Weight loss is also common in people with cancer, and it can continue during treatment. Some patients experience reduced appetite, nausea, or other side effects that can lower weight. So weight gain is possible, but weight loss is also frequently seen during cancer treatment.
When should weight gain be checked with a clinician?
Weight gain should be discussed promptly if it is rapid, significant, or accompanied by symptoms that could suggest an immune-related problem, such as:
- unusual fatigue or feeling cold (possible hypothyroidism)
- swelling or changes in energy levels
- new or worsening appetite changes
Clinicians often monitor thyroid function tests during treatment, which helps catch hormone-related causes early.
What can patients do if they gain weight on Keytruda?
Patients typically start with medical review rather than trying to “treat” the weight gain themselves. The practical next steps usually include:
- checking recent labs (especially thyroid tests)
- reviewing other medications that might affect weight
- discussing nutrition and activity goals that match the person’s cancer status and side effects
If an immune-related endocrine issue is found, treatment (for example, thyroid hormone replacement for hypothyroidism) can help reverse symptoms, including weight changes.
Is there a pattern to when weight gain happens?
There isn’t a single universal timeline. Weight change can appear at any point during treatment depending on the cause. Hormone-related effects, when they occur, often develop over weeks to months, so symptoms may emerge after treatment starts.
Source
No direct citation was provided in the information available to me for this question. If you share the context (age, other conditions, thyroid history, and how long you’ve been on Keytruda), I can help you interpret whether your weight change sounds consistent with a likely cause and what questions to ask your clinician.