Why Do Targeted Cancer Drugs Stop Working Over Time? (As of 2026)

Resistance develops because the very gene mutation that a targeted therapy was designed to attack changes again within the cancer cells, or the cancer finds an alternate pathway around it—and when this happens, the standard course is to retest and move on to the next drug.

Key Takeaways

• Targeted therapies are drugs designed to attack specific molecules that are overexpressed or mutated in cancer cells. The choice of drug depends on markers such as EGFR, ALK, and ROS1 in lung cancer; HER2 and CLDN18.2 in gastric cancer; HER2 and hormone receptors in breast cancer; and RAS and BRAF in colorectal cancer (National Cancer Center Korea).
• Targeted therapies can also develop resistance over time. When this happens, the standard course of action is to retest to identify what has changed in the tumor, then move on to the next drug.
• If a patient does not carry the relevant mutation, targeted therapy is not an option, and cytotoxic chemotherapy, immunotherapy, surgery, or radiation become the standard approach instead.
• The NCCN patient guidelines describe biomarker testing—both before starting treatment and when resistance develops—as a standard procedure across many cancer types.
• Treatment response is generally assessed by CT scan every 2 to 3 months, and as a rule, treatment continues as long as the drug remains effective.

Why does a drug that works at first eventually stop working?

• Targeted therapy works by blocking one specific faulty point in the growth signaling pathway that drives cancer cell growth. Over time, however, cells within the tumor population that have altered that target or found a way around it through another pathway survive and continue to grow, which is how resistance develops.
• Cancer arises when the growth signals of normal cells malfunction, and the exact location of that malfunction differs from tumor to tumor. Since a drug only blocks one specific faulty point, once that point changes, the same drug no longer works.
• This process is understood as a natural phenomenon in which cancer cells change under selective pressure during treatment. It does not occur at the same time point in every patient—individual variation is considerable, and the timing and pattern of resistance differ from drug to drug.

What steps are taken when resistance develops?

1. Step 1 — Response assessment: Changes in tumor size are typically checked by CT scan every 2 to 3 months, and as a rule, treatment continues for as long as the drug remains effective.
2. Step 2 — Retesting: If imaging confirms disease progression, genetic testing is repeated using tissue or blood samples. When obtaining tissue is difficult, a blood-based method that examines circulating tumor DNA may be used instead.
3. Step 3 — Deciding on the next drug: Once retesting identifies what has changed, the general course is to move on to a different targeted therapy, cytotoxic chemotherapy, or immunotherapy, depending on the cancer type and the nature of the mutation.
4. Because the specific new mutation that emerges and the available next options vary from person to person, the decision is ultimately based on the judgment of the treating medical team.

Is there a targeted drug that never develops resistance?

• Not all targeted therapies develop resistance at the same rate, and differences exist between drugs, but no targeted therapy can be said to completely rule out the possibility of resistance.
• Targeted drugs broadly come in three forms: oral inhibitor pills, intravenous antibody infusions, and antibody-drug conjugates that attach a chemotherapy agent to an antibody. Regardless of form, the potential to develop resistance is a shared characteristic across all of them.
• However, the onset of resistance does not mean treatment has ended—it is followed by a process of retesting to identify the next target or an alternative treatment approach.

FAQ — Do side effects change when resistance develops and the drug is switched?

Yes, the pattern of side effects can change when the drug changes. EGFR-class drugs commonly cause skin rash, diarrhea, and inflammation around the nails; HER2-targeted therapy requires monitoring of heart function; and anti-angiogenic drugs require watching for elevated blood pressure, bleeding, and delayed wound healing. When switching to a new drug, these signs need to be monitored freshly all over again.

FAQ — How do the side effects of targeted therapy differ from those of chemotherapy?

Hair loss and severe nausea are relatively less common with targeted therapy, but rash, diarrhea, elevated blood pressure, and reactions affecting the heart, liver, or lungs can occur, which requires regular testing. This principle applies equally after a drug switch due to resistance, and the overall pattern of reactions is described as distinct from that of cytotoxic chemotherapy.

Sources and Notes

• This article was prepared based on notes on targeted therapy and immuno-/targeted-therapy side effects, drawing on materials from the National Cancer Center Korea, the NCCN patient guidelines, and the U.S. National Cancer Institute (NCI).
• This content is intended for general medical information purposes only. Diagnosis, treatment methods, and outcomes may vary depending on individual circumstances. Accurate diagnosis and treatment should always be determined through consultation with a medical professional.