Keytruda, also known as pembrolizumab, is an immune checkpoint inhibitor medication used to treat various types of cancer. While Keytruda has been proven effective in treating these conditions, it can also cause several side effects. Some side effects require immediate medical attention.

According to the Keytruda Prescribing Information, serious side effects include:

1. **Immune-Mediated Pneumonitis:** This is inflammation of the lungs that can cause symptoms such as cough, chest pain, and difficulty breathing. If left untreated, it can be life-threatening.

2. **Colitis:** This is inflammation of the colon, which can cause symptoms such as diarrhea, abdominal pain, and blood in the stool. Severe colitis can lead to tears in the colon, a condition known as perforation.

3. **Hepatitis:** This is inflammation of the liver, which can cause symptoms such as jaundice (yellowing of the skin and eyes), dark urine, and fatigue. Severe hepatitis can lead to liver failure.

4. **Endocrinopathies:** These are hormonal imbalances that can cause symptoms such as fatigue, weight gain or loss, and changes in mood or behavior. Severe endocrinopathies can lead to adrenal crisis or diabetic ketoacidosis, both of which are life-threatening conditions.

5. **Nephritis:** This is inflammation of the kidneys, which can cause symptoms such as decreased urine output, swelling in the legs and ankles, and fatigue. Severe nephritis can lead to kidney failure.

6. **Skin Reactions:** These can range from mild rashes to severe reactions such as Stevens-Johnson syndrome and toxic epidermal necrolysis, both of which can be life-threatening.

7. **Other Immune-Mediated Reactions:** These can affect any organ system in the body and can cause a variety of symptoms.

If you experience any of these side effects while taking Keytruda, it is crucial to seek immediate medical attention. It is also important to note that this is not an exhaustive list of all possible side effects of Keytruda. For a full list, please refer to the Keytruda Prescribing Information.

Sources:
[1] Keytruda (pembrolizumab) Prescribing Information. Merck & Co., Inc. 2021. Available from: <https://www.merck.com/product/usa/pi_circulars/k/keytruda/keytruda_pi.pdf>
[2] DrugPatentWatch.com. Keytruda (pembrolizumab) - Patent, Biosimilar & Competitor Information. Available from: <https://www.drugpatentwatch.com/keytruda-pembrolizumab/>