What the label lists and what reports show
Mechlorethamine side effects
Mechlorethamine's FDA label: “The most common adverse reactions (≥5%) are dermatitis, pruritus, bacterial skin infection, skin ulceration or blistering, and hyperpigmentation ( 6.1 ).” FAERS holds 2,244 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is pruritus (322 reports).
- No generic listed
Check it at the source: the mechlorethamine FDA label on DailyMed (effective November 20, 2024, Helsinn Therapeutics (U.S.), Inc.); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of VALCHLOR?See " What is the most important information I should know about VALCHLOR?" VALCHLOR can cause serious side effects, including: Risk of Secondary Exposure to VALCHLOR. You may have a risk of inflammation of your skin (dermatitis), injury to your eyes, mouth, or nose, and certain types of cancers. Caregivers who accidentally come into contact with VALCHLOR must wash the affected area with soap and water right away for at least 15 minutes and remove any contaminated clothing. Get medical help right away if VALCHLOR gets in your eyes, mouth, or nose. Inflammation of your skin (dermatitis) is common with VALCHLOR and may sometimes be severe. Your risk for dermatitis is increased if VALCHLOR is applied to your face, genital area, anus, or skin folds. Tell your healthcare provider if you develop skin reactions such as redness, swelling, itching, blisters, ulcers, and skin infections. […]
Source: FDA drug label, Helsinn Therapeutics (U.S.), Inc., effective November 20, 2024 (SPL set 6a52e4c2-6a9f-4ebb-bc77-046b4f8bcd57), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reactions (≥5%) are dermatitis, pruritus, bacterial skin infection, skin ulceration or blistering, and hyperpigmentation ( 6.1 ).
Table 1. Most Commonly Reported (≥5%) Cutaneous Adverse Reactions
| Reaction | Any Grade | Moderately-Severe or Severe | Any Grade | Moderately-Severe or Severe |
|---|---|---|---|---|
| Dermatitis | 56 | 23 | 58 | 17 |
| Pruritus | 20 | 4 | 16 | 2 |
| Bacterial skin infection | 11 | 2 | 9 | 2 |
| Skin ulceration or blistering | 6 | 3 | 5 | 2 |
| Skin hyperpigmentation | 5 | 0 | 7 | 0 |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe following clinically significant adverse reactions are discussed in greater detail in other sections of the prescribing information: Mucosal or eye injury [ see Warnings and Precautions ( 5.1 ) ] Secondary exposure to VALCHLOR [ see Warnings and Precautions ( 5.2 ) ] Dermatitis [ see Warnings and Precautions ( 5.3 ) ] Non-melanoma skin cancer [ see Warnings and Precautions ( 5.4 ) ] The most common adverse reactions (≥5%) are dermatitis, pruritus, bacterial skin infection, skin ulceration or blistering, and hyperpigmentation ( 6.1 ). To report SUSPECTED ADVERSE REACTIONS, contact Helsinn Therapeutics (U.S.), Inc., at 1-855-482-5245 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared with rates in the clinical trials of another drug and may not reflect the rates observed in clinical practice. In a randomized, observer-blinded, controlled trial, VALCHLOR 0.016% (equivalent to 0.02% mechlorethamine HCl) was compared to an Aquaphor ® -based mechlorethamine HCl 0.02% ointment (Comparator) [ see Clinical Studies ( 14 ) ]. The maximum duration of treatment was 12 months. Sixty-three percent (63%) of patients in the VALCHLOR arm and 67% in the comparator arm completed 12 months of treatment. …
Source: FDA drug label, Helsinn Therapeutics (U.S.), Inc., effective November 20, 2024 (SPL set 6a52e4c2-6a9f-4ebb-bc77-046b4f8bcd57), via openFDA; the full label on DailyMed
What do FAERS reports show?
2,244 reports list mechlorethamine among the medicines taken, each a report of something that happened, not proof the medicine caused it; 154 are coded as a death and 248 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 19 reactions below, 2 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Mechlorethamine: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions