Weight Gain, Constipation and a Slow Pulse — on Alectinib

There is no single figure that separates normal from concerning, and that framing is less useful than watching the pattern. Gradual gain over weeks to months, without swelling or breathlessness, is the typical picture and can be monitored at home. Sudden or rapid gain — particularly if your ankles, legs or face swell — is the signal to call your team, because it can indicate fluid accumulating somewhere it should not. Keeping a daily weight record at the same time each morning is the most reliable way to catch a change early and give your team useful information at appointments.

Your team will tell you your own baseline and the range they consider acceptable for you, because the threshold is individual rather than a single number applying to everyone. What the number alone does not tell you is as important as what it says: a pulse that looks low on a monitor but causes no symptoms is managed differently from the same reading alongside dizziness, faintness or breathlessness. Report any symptoms you notice rather than waiting to see whether a specific number is reached. Your team monitors your pulse and ECG at appointments for exactly this reason.

Ask your oncology team or clinical pharmacist before buying anything over the counter, as some types suit drug-related constipation better than others. Starting with simple measures — more water, more fibre, daily gentle walking — is reasonable while you wait to ask. Do not take stimulant laxatives without guidance, as they can cause cramping and are not the right first step for this type of constipation. Your team can recommend something appropriate for your situation.

No. Weight gain is a side effect of how alectinib affects fluid balance and, in some patients, thyroid hormone levels. It does not reflect what is happening to the tumour. Response to treatment is assessed through scans and clinical review, not through weight change. If you are anxious about how well the treatment is working, your next scan result or clinic appointment is the right place to raise that question directly.

Do not stop without speaking to your team first. Alectinib is a targeted treatment taken continuously, and stopping it without guidance can allow the cancer to progress. Most side effects at the manageable end of the scale are handled while the treatment continues — sometimes with a short dose pause or adjustment if the symptom is significant. Stopping on your own removes that option. If the side effect is in the red-flag category on this page, seek emergency care — but that is a different situation from managing a common, mild symptom.