A new toxicological analysis has revealed that St. Thérèse of Lisieux suffered mercury poisoning shortly before her death. According to Professor Philippe Charlier – a forensic pathologist heading the laboratory at the University of Paris-Saclay – the findings were announced on Tuesday, May 20, 2025. Until now it was understood that Thérèse died of foudroyant (acute) tuberculosis on September 30, 1897, at age 24. This “Little Flower,” already weakened by losing a lung and enduring great suffering, succumbed to that disease.
However, Charlier’s study, published in connection with the centenary of the Herbitzheim pilgrimage, has uncovered an additional factor: Thérèse’s body shows clear evidence of mercury intoxication. (If you're wondering where is Herbitzheim, it is a commune in Alsace in the north east of France, and is known as a “little Lisieux." It has held an annual pilgrimage to St. Thérèse each Pentecost since 1925.)
The investigation involved the analysis of a lock of Thérèse’s hair. With permission from the Archdiocese of Strasbourg, Prof. Charlier took two strands of hair from one of her relics in the Herbitzheim church and sent them for examination in Denmark. As the professor shared with Aleteia, one strand was unusable, but the other yielded remarkable new insights.
Dating of the hair segment shows it corresponds exactly to the last four weeks of Thérèse’s life. The forensic report notes a dramatic “agonizing process” in those final weeks, evidenced by a sharp drop in iron and copper concentrations in her hair. (This reflects her body’s heavy strain as she lay dying.) Most strikingly, the hair shows two distinct spikes of mercury about 10 days before she died.
Mercury itself is a heavy metal, toxic to the body in large amounts. In 1897 it was commonly used in certain medical treatments – for example, some remedies for intestinal parasites or, notably, for tuberculosis. Charlier explains that the Carmelite doctor in Lisieux likely administered a mercury-based medicine in an attempt to cure Thérèse.
Accelerating the decline
Unfortunately, this appears to have worsened her condition. “We can therefore say that Saint Thérèse died of tuberculosis, but also of an iatrogenic complication (induced by the medication or the doctor),” he observes.
In other words, she still would have died of TB, but the mercury treatment – given in an era with no antibiotics – accelerated her final decline. “She had already lost one lung and was suffering martyrdom,” Charlier notes, and TB was the great scourge of the 19th century until modern drugs. The mercury may have been a doomed attempt to save her, but it likely hastened her death.
These scientific findings, based on careful hair analysis, do not change Thérèse’s sanctity but rather shed light on the historical circumstances of her suffering.
As Catholics, it is possible to take heart that even in this grim scientific detail, God’s providence was at work in St. Thérèse’s life. Prof. Charlier himself admitted he was deeply moved by Thérèse’s witness.
“I did not know Saint Thérèse of Lisieux intimately. To better understand the young Carmelite who became my 'patient,' I read Story of a Soul,” he said. “Like many, I found it difficult not to be touched by this figure whose words have universal meaning."
In fact, May 17, 2025, happened to be the 100th anniversary of Thérèse’s canonization, so this medical revelation is a fitting moment to reflect on her legacy.
Saint Thérèse’s own “Little Way” of spiritual childhood offers profound insight. Far from grand mystical feats, she pursued holiness in small acts of love and complete trust in God. As she herself wrote, “I am too small to climb the rough ladder of perfection… I want to seek out a tiny, straight, very short way” – a “new little path” by which God alone raises us. Crucially, she concluded, “only love moves the members of the Church."
In other words, it is not spectacular deeds but loving surrender in ordinary life that most pleases God. Even in her illness, St. Thérèse offered up her pains as tiny flowers of love for Jesus, confident in His mercy.
A lesson in faith
Indeed, St. Thérèse teaches us that we can approach God with the trust of little children. Pope St. John Paul II noted that Thérèse’s spirituality is essentially a “way of spiritual childhood,” reminding us that “God is our Father and we are His children,” as reported in Catholic Culture. She lived with a simple confidence: God looks especially for the weak and imperfect and accompanies them on the path of holiness.
As one reflection, shared by Little Flower Basilica, on her teaching explains, we should admit our littleness and trust God to “carry us off to His Kingdom” as we cannot help ourselves.
In her own words, Thérèse surrendered herself entirely to God’s will, believing that her Father would never abandon her. We need only do our small part and “confidently surrender the rest to [Him] who is always faithful and who will never leave nor forsake us."
On this centenary of her canonization, St. Thérèse’s witness shines especially bright. Her final sufferings – now understood even more fully by science – were born in complete union with God’s will. Even as the mercury in her system increased her pain, she remained “like a little child” trusting in her Father’s care.
She once offered the prayer, “Jesus, I surrender myself to You; take care of everything” – a model for us in illness and trial. May her example and intercession strengthen our trust. As we honor the memory of St. Thérèse of Lisieux, let us follow her “Little Way:” offering every hardship to God in love, confident that “God is love” and will work all things for the good of His children.
In this Jubilee Year of Hope, look to some wise words from St. Thérèse and the Bible to inspire you:
