What are the side effects of Alpelisib?
Last updated September 7, 2026.
**The most common side effects of alpelisib are high blood sugar, rash, diarrhea, nausea, mouth sores, low appetite, weight loss, and fatigue.** Alpelisib blocks a signaling protein called PI3K, which also helps your body handle glucose, so raised blood sugar is the effect your care team watches most closely. Your fasting glucose and A1C are checked before you start and on a set schedule afterward, and many people take metformin alongside it. Rash usually shows up in the first two weeks, and starting a non-sedating antihistamine on day one lowers how often it happens. Taking the tablet with food, drinking fluids steadily, and keeping a simple log of glucose readings, stools, and mouth soreness gives your team what it needs to adjust the dose before a problem grows.
What to do
- Track your glucose: Check blood sugar as instructed and report readings that stay high, along with thirst, frequent urination, or blurry vision.
- Start rash prevention early: Ask whether to begin a non-sedating antihistamine on your first day, since rash usually appears within the first two weeks.
- Treat diarrhea fast: Keep an antidiarrheal your team approves on hand, drink fluids, and call if stools stay loose for more than a day.
- Care for your mouth: Rinse with a salt and baking soda solution several times daily and skip alcohol-based mouthwash to limit sores.
- Check every new medicine: Tell your pharmacist before adding any drug, supplement, or herbal product, because several interact with alpelisib.
- Know the urgent signs: Seek care the same day for blistering or peeling skin, fever with rash, facial swelling, or new breathing trouble.
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