Most Enhertu side effects follow a predictable pattern: nausea and digestive symptoms peak in the first week after each infusion and ease before the next cycle, fatigue builds gradually over months of treatment, and hair thinning is typically reversible within three to six months after stopping. The timeline varies depending on which side effect you’re dealing with, so here’s what to expect for each one.
Enhertu (trastuzumab deruxtecan) is given as an IV infusion every three weeks. Because it delivers a chemotherapy payload directly to cancer cells, many of its side effects mirror traditional chemo, though the intensity and duration differ. In large clinical trials involving over 1,000 patients, nausea affected 72 to 77% of people, fatigue hit 31 to 48%, hair thinning occurred in 37 to 40%, and low blood counts developed in roughly a third of patients.
Nausea and Vomiting: The First Week
Nausea is the single most common side effect, and it tends to show up early. Researchers tracking nausea patterns define three phases: an immediate phase during the first 24 hours after infusion, a delayed phase from days one through five, and an extended-delayed phase stretching out to about seven days (168 hours). For most people, nausea is worst in the first few days and gradually fades before the next three-week cycle.
That said, Enhertu-related nausea can appear from the very first cycle and persist across treatment. The National Comprehensive Cancer Network reclassified Enhertu as highly emetogenic in 2023, meaning it carries a high risk of causing nausea and vomiting. This prompted a shift to more aggressive anti-nausea medication from the start, typically a three-drug regimen given around each infusion. With proper prevention, many patients find nausea manageable, but if the first cycle hits hard, your oncology team can adjust the approach for cycle two.
Vomiting is less common than nausea, affecting 34 to 52% of patients across trials, and severe vomiting requiring medical intervention occurs in fewer than 4% of cases. Both nausea and vomiting tend to follow the same weekly arc, peaking and resolving within each cycle rather than building continuously over time.
Fatigue: A Cumulative Effect
Fatigue affects roughly one in three to one in two patients on Enhertu, depending on the study. Unlike nausea, which cycles with each infusion, fatigue often builds cumulatively over several months of treatment. Most people experience mild to moderate tiredness. Severe fatigue that significantly limits daily activity occurs in about 5 to 8% of patients.
There’s no clean “day one through day five” window for fatigue the way there is for nausea. Many patients describe feeling more drained as treatment continues, with partial recovery between cycles but a gradually lower baseline. After stopping Enhertu, fatigue typically improves over weeks to a few months, though recovery speed depends on how long treatment lasted and your overall health.
Hair Thinning and Loss
Hair thinning from Enhertu is different from the rapid, total hair loss associated with some traditional chemotherapy regimens. In clinical trials, 37 to 40% of patients experienced alopecia, and nearly all cases were mild, meaning diffuse thinning rather than complete baldness. Severe hair loss was reported in fewer than 1% of patients.
Thinning typically starts one to three weeks after beginning treatment and can progress with continued cycles. The good news: this type of hair loss is generally reversible. Regrowth usually begins three to six months after treatment ends. The new hair may initially differ in texture or color from what you had before, but it typically returns to normal over time.
Low Blood Counts
Enhertu can suppress your bone marrow’s ability to produce blood cells, leading to low white blood cells (neutropenia), low red blood cells (anemia), and low platelets (thrombocytopenia). In trials, neutropenia occurred in 31 to 36% of patients, with severe drops in 14 to 18%. Anemia affected 28 to 37%, with severe cases in 8 to 9%.
Blood counts are checked before every infusion. When counts drop too low, your treatment may be paused for one or more cycles to let your marrow recover. Most blood count changes resolve within one to two weeks of a treatment break, though recovery time varies. If low counts become a recurring issue, your dose may be permanently reduced. Once Enhertu stops, blood counts typically normalize within a few weeks.
Lung Problems: ILD and Pneumonitis
Interstitial lung disease (ILD) and pneumonitis are the most serious potential side effects of Enhertu, serious enough to warrant a boxed warning on the label. This is inflammation in the lungs that can cause cough, shortness of breath, and fever. It affected 10 to 15% of patients in major trials, and while most cases were mild, fatal outcomes occurred in a small number (under 1%).
The timeline for lung problems is very different from other side effects. Rather than showing up in the first week, ILD has a median onset of about 4 to 6 months into treatment, with a wide range from as early as one month to nearly two years. In the DESTINY-Breast03 trial, the median time to onset was 5.5 months. In DESTINY-Breast04, it was approximately 4.3 months.
How long ILD lasts depends entirely on severity and how quickly it’s caught. Mild cases (no symptoms, found only on imaging) may resolve over weeks with steroid treatment while Enhertu is paused. For symptomatic cases, steroids are typically continued for at least 14 days and then tapered over four or more weeks. About 70% of ILD cases in clinical trials resolved or were actively resolving with treatment. However, any ILD graded as symptomatic means Enhertu is permanently discontinued, so for those patients, recovery from lung inflammation becomes the priority rather than resuming therapy.
Symptoms to watch for throughout treatment include a new or worsening cough, difficulty breathing, and unexplained fever. These warrant prompt contact with your oncology team, because early detection makes a significant difference in outcomes.
What Happens With Dose Reductions
When side effects become severe or persistent, your oncologist may reduce the dose or temporarily pause treatment. Enhertu’s prescribing information outlines a stepwise dose reduction schedule, and once a dose is lowered, it stays at the reduced level for the rest of treatment. Many patients continue on a lower dose with meaningfully fewer side effects. In published case reports, patients who received a 25% dose reduction saw improvements in lab values and symptoms within about two to three weeks.
Treatment interruptions for side effects are common and built into the management plan. A pause of one or two cycles often allows blood counts to recover and nausea or fatigue to ease before resuming. These breaks don’t necessarily mean treatment is failing. They’re a standard part of how Enhertu is managed over the long term.
After Treatment Ends
Most side effects tied to Enhertu’s chemotherapy payload, including nausea, blood count changes, and appetite loss, resolve within a few weeks of the final infusion. Hair regrowth begins in the three-to-six-month window. Fatigue can linger longer, sometimes for several months, particularly if you were on treatment for an extended period. Lung inflammation, if it occurred, follows its own recovery arc dictated by steroid treatment and severity, potentially taking weeks to months to fully resolve.