Ado-trastuzumab emtansine: HER2 breast cancer, dosing, side effects
Last updated September 7, 2026.
Ado-trastuzumab emtansine is an antibody-drug conjugate, a targeted cancer treatment that links the HER2 antibody trastuzumab to the chemotherapy agent DM1. The antibody part attaches to HER2 proteins on the surface of breast cancer cells, and the cell then pulls the drug inside, where DM1 breaks down the internal scaffolding the cell needs to divide. This delivers chemotherapy mostly to HER2-positive cells rather than throughout the body. The most important safety point is that ado-trastuzumab emtansine is not interchangeable with plain trastuzumab; the two are different medicines at different doses, and swapping them causes serious harm, so the full name is written and read back at every infusion.
What does it treat?
Ado-trastuzumab emtansine treats breast cancer that tests HER2-positive, meaning the tumor makes too much of the HER2 growth protein. It is used in two situations. The first is early-stage breast cancer when you received chemotherapy with a taxane plus trastuzumab before surgery and the surgical specimen still showed invasive cancer; ado-trastuzumab emtansine is then given after surgery to lower the chance the cancer returns. The second is HER2-positive breast cancer that has spread beyond the breast and lymph nodes in people who already received trastuzumab and a taxane, either together or at separate times. It is given by itself, not mixed into a combination regimen, and it is sold under the brand name Kadcyla. It does not treat HER2-negative or hormone-receptor-only breast cancer, which is why the pathology report drives the decision.
How do you take it?
Ado-trastuzumab emtansine comes only as a powder that a pharmacy mixes and a nurse infuses into a vein, so there is no pill and no home dose to measure. The adult dose is 3.6 mg per kilogram of body weight given every 3 weeks, and no single dose goes above 3.6 mg/kg. The first infusion runs over 90 minutes with 90 minutes of watching afterward; if you tolerate it, later infusions run over 30 minutes with 30 minutes of watching. After surgery for early breast cancer the course is 14 cycles unless the cancer returns or side effects become unmanageable; for cancer that has spread, treatment continues while it is working and tolerated. If side effects such as a rise in liver enzymes, a low platelet count, or a drop in heart pumping function show up, your oncologist steps the dose down to 3 mg/kg and then to 2.4 mg/kg, and stops the drug rather than going lower. Blood counts and liver tests are drawn before every dose, and heart function is checked before the first dose and about every 3 months. There are no food rules because the drug bypasses the stomach, but skip grapefruit and tell your team about any strong CYP3A4 blockers. If you miss a scheduled infusion, do not wait for the next planned cycle; call the clinic and get the dose as soon as you can, and the every-3-week clock restarts from that day. Call before the next infusion if you notice yellowing of the eyes, easy bruising or bleeding, a cough that will not settle, or new shortness of breath.
Side effects to know
- Common: Most people notice fatigue, nausea, headache, constipation, muscle or joint aches, nosebleeds, and blood tests showing higher liver enzymes or a lower platelet count, and some develop tingling or numbness in the hands and feet.
- Serious: Stop and get medical help for signs of liver injury such as yellow eyes or skin, dark urine, right upper belly pain, or confusion; for heart failure signs such as new shortness of breath, a cough at night, swollen ankles, or rapid weight gain; and for lung inflammation, which shows up as a dry cough and breathlessness that keeps getting worse. Bleeding that will not stop, a severe headache, black or bloody stools, a hypersensitivity reaction with hives, facial swelling, or wheezing during an infusion, and burning or swelling at the IV site all need immediate attention.
Who should not take it?
Ado-trastuzumab emtansine is not for anyone whose cancer is not HER2-positive, and it must never be given in place of, or under the name of, trastuzumab. Tell your oncologist if you have liver disease or hepatitis, since the drug carries a boxed warning for liver injury and has not been studied in severe liver impairment; if you have heart failure, a low ejection fraction, uncontrolled high blood pressure, or a prior heart attack; if you have had interstitial lung disease, pneumonitis, or lung damage from radiation; if you have a bleeding disorder or a low platelet count; or if you take a blood thinner such as warfarin, apixaban, or aspirin, which raises bleeding risk. Strong CYP3A4 blockers including ketoconazole, itraconazole, clarithromycin, and ritonavir should be avoided because they raise DM1 levels; ask before starting any new medicine, including over-the-counter products and supplements. The drug can harm a developing baby, so pregnancy is confirmed as negative before treatment, women who can become pregnant use reliable birth control during treatment and for 7 months after the last dose, and men with partners who can become pregnant use condoms during treatment and for 4 months after. Do not breastfeed during treatment or for 7 months after the last dose. There is no fixed alcohol rule on the label, but because liver injury is the headline risk and liver enzymes are tracked at every visit, keep alcohol to a minimum or skip it and ask your oncologist what is reasonable for you.
When is it an emergency?
Call 911 in the United States if you collapse, cannot catch your breath, have chest pain, have swelling of the lips, tongue, or throat with hives or wheezing during or after an infusion, cough up blood, vomit blood or material that looks like coffee grounds, have bleeding that will not stop with pressure, or have a sudden severe headache, weakness on one side, or confusion, which can signal bleeding in the brain. Go to an emergency department the same day for yellow eyes or skin with belly pain, a fever of 100.4 F or higher, or shortness of breath that is worse than yesterday. If more than the prescribed amount is infused or the wrong product is given, tell the infusion team immediately and call Poison Control at 1-800-222-1222; there is no antidote, an overdose has been linked to a dangerously low platelet count and to a death, and you will need close monitoring and repeat blood counts. Save the infusion record or label so the team can confirm the exact product and dose.
What a Pymander AI doctor consult looks like
Illustrative example, not a real member's messages.
Common questions
What is the maximum dose of ado-trastuzumab emtansine?
The dose is 3.6 mg per kilogram of body weight given into a vein every 3 weeks, and no single dose exceeds 3.6 mg/kg. Your weight is rechecked so the dose is recalculated when it changes. If side effects come up, the dose goes down in steps to 3 mg/kg and then 2.4 mg/kg, and doses are never raised again after a reduction. If a further reduction would be needed, the drug is stopped instead. For early breast cancer after surgery, the course runs 14 cycles.
Can I drink alcohol during treatment?
The label does not ban alcohol, but liver injury is the main boxed warning for this drug and your liver enzymes are measured before every infusion. Alcohol adds a second stress on the same organ and can make the enzyme numbers rise, which may delay or reduce your dose. The practical approach most oncologists take is no alcohol or an occasional single drink at most. Ask your own oncologist, since the answer changes if your enzymes are already elevated or you have hepatitis or fatty liver disease.
Is it safe during pregnancy or breastfeeding?
No. Ado-trastuzumab emtansine can cause fetal harm and death, including kidney damage and low amniotic fluid, so pregnancy is ruled out with a test before treatment starts. If you can become pregnant, use reliable birth control during treatment and for 7 months after the last dose; if you are a man with a partner who can become pregnant, use condoms during treatment and for 4 months after. Tell your oncologist right away if you become pregnant during treatment. Do not breastfeed while receiving the drug or for 7 months after the final dose.
Do I need to change what I eat or avoid any foods?
There are no food restrictions, because the drug goes into a vein and never passes through your stomach. Eating a light meal before an infusion often helps with nausea, and your team can give anti-nausea medicine if you need it. Skip grapefruit and grapefruit juice, since they block the same liver pathway that clears DM1 and can raise your exposure. Constipation is common, so fluids and fiber help; ask before starting any herbal supplement, including St. John's wort.
What are the signs of getting too much?
The clearest sign of an overdose is a crashing platelet count, which shows up as easy bruising, nosebleeds that will not stop, bleeding gums, tiny red dots on the skin, or black stools. Reported overdoses have caused severe low platelets and one death. There is no antidote, so treatment is close monitoring, blood counts, and supportive care such as platelet transfusion if needed. Tell the infusion team immediately if you think the wrong dose or the wrong product was given, and call Poison Control at 1-800-222-1222. Call 911 for heavy bleeding, fainting, or trouble breathing.
What medicines interact with ado-trastuzumab emtansine?
Avoid strong CYP3A4 blockers such as ketoconazole, itraconazole, voriconazole, clarithromycin, and ritonavir, because they slow the breakdown of DM1 and raise the risk of toxicity; if one is unavoidable, your oncologist will delay the dose or watch you closely. Blood thinners and antiplatelet drugs including warfarin, apixaban, clopidogrel, aspirin, and regular ibuprofen add to the bleeding risk from low platelets. Other liver-stressing drugs, including high-dose acetaminophen and some statins, matter because liver enzymes are already being tracked. Bring a full list of prescriptions, over-the-counter products, and supplements to every visit.