The International Agency for Research on Cancer, the specialised cancer agency of the World Health Organization, has classified three widely used medicines as carcinogenic to humans. Hydrochlorothiazide, voriconazole and tacrolimus have been placed in Group 1, the highest category used by the agency to indicate that there is sufficient evidence of a cancer hazard in people. The evaluations appear in Volume 137 of the IARC Monographs and were finalised after a scientific working group reviewed epidemiological studies, animal data and mechanistic evidence.
What Group 1 Classification Means
IARC classifications identify whether an agent can cause cancer. They do not measure how great the risk is for any individual, nor do they weigh benefits against harms. Group 1 means the agency found sufficient evidence that the substance is carcinogenic to humans. Many everyday exposures and necessary medical treatments fall into this category. The finding is intended to inform regulators, clinicians and researchers rather than to serve as a direct instruction for patients.
In the case of these three drugs, the primary route of exposure is through prescribed medication. Environmental or occupational contact is expected to be far lower.
Hydrochlorothiazide
Hydrochlorothiazide is a thiazide diuretic commonly prescribed for essential hypertension. It is often used alone or in fixed-dose combination tablets with other blood-pressure medicines. Despite the availability of newer agents, it remains widely used because high blood pressure is so prevalent worldwide.
The working group found sufficient evidence that hydrochlorothiazide causes squamous cell carcinoma of the skin and cancer of the lip in humans. Some additional associations with other skin cancers were judged limited. The classification rests mainly on human epidemiological data supported by findings in experimental animals.
Voriconazole
Voriconazole is a broad-spectrum triazole antifungal medicine used to treat serious invasive fungal infections, particularly invasive aspergillosis. It is frequently given to transplant recipients and other immunocompromised patients, both for treatment and, in some cases, for prevention.
IARC concluded there is sufficient evidence that voriconazole causes squamous cell carcinoma of the skin in humans. Strong mechanistic evidence, including effects observed in combination with ultraviolet radiation, further supported the Group 1 evaluation. The drug’s ability to increase photosensitivity is one of the pathways considered relevant to the observed skin cancers.
Tacrolimus
Tacrolimus is a potent immunosuppressive agent. Its main uses are to prevent organ rejection in solid-organ transplant recipients and to prevent graft-versus-host disease after certain stem-cell transplants. A topical form is also used as second-line treatment for some skin conditions such as atopic dermatitis.
The agency found sufficient evidence that tacrolimus causes non-Hodgkin lymphoma and post-transplant lymphoproliferative disorder. Evidence for leukaemia and squamous cell carcinoma of the skin was considered limited. For tacrolimus the Group 1 classification was reached both through sufficient human evidence and through the combination of sufficient evidence in experimental animals plus strong mechanistic evidence in exposed humans.
Clinical Context and Patient Advice
These medicines treat serious or life-threatening conditions. High blood pressure, invasive fungal infections and the risk of transplant rejection carry substantial dangers of their own. In many clinical situations the benefits of continued treatment clearly outweigh the potential cancer risks identified by IARC.
Health authorities and clinicians emphasise that patients should not stop or change any prescribed medicine on the basis of the classification alone. Decisions about therapy belong with the treating physician, who can weigh individual risk factors such as duration of use, cumulative dose, skin type, sun exposure, immune status and the availability of alternatives. Regular skin checks, sun protection and adherence to monitoring protocols remain important for people taking these drugs, especially those on long-term or high-dose regimens.
How the Evaluations Were Reached
An international working group of scientists examined published epidemiological studies, cancer bioassays in laboratory animals and mechanistic research. For each agent they assessed the strength of evidence across these streams and reached a consensus classification. The full monographs provide detailed summaries of the data and the reasoning behind the conclusions. The summary of the evaluations was first communicated in late 2024, with the complete Volume 137 made available subsequently.
Implications Beyond Individual Patients
The classifications may influence regulatory labelling, clinical guidelines and future research priorities. Drug regulators already require warnings and risk-management measures for many of these agents; the IARC findings add an independent scientific assessment of the carcinogenic hazard. Researchers may explore safer alternatives, refined dosing strategies or enhanced surveillance protocols. Public-health agencies can use the information when updating educational materials for both clinicians and patients.
At the same time, the evaluations illustrate a broader principle: many essential medicines carry known risks, including the possibility of cancer, yet remain indispensable. Transparent communication of both hazards and benefits is necessary so that informed decisions can be made.
A Balanced Perspective
The placement of hydrochlorothiazide, voriconazole and tacrolimus in Group 1 is a significant scientific determination based on the available evidence. It identifies a cancer hazard associated with these agents when used as medicines. It does not mean that every person who takes them will develop cancer, nor does it imply that the drugs should be withdrawn. For the large majority of patients the therapeutic necessity of these treatments continues to guide clinical practice.
Anyone currently prescribed one of these medicines who has concerns should discuss them with their doctor. Abrupt discontinuation can create greater immediate risks than the long-term hazard identified by the cancer agency. Ongoing medical supervision, appropriate monitoring and lifestyle measures such as sun protection offer practical ways to manage the identified risks while preserving the benefits of treatment.
The IARC monographs serve as a rigorous, independent evaluation of carcinogenic hazards. In the case of these three widely used drugs, they provide clarity for the scientific and medical communities while reinforcing the importance of individualised clinical judgement for every patient.
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