Sweet syndrome is a skin condition marked by the sudden eruption of painful red bumps, nodules, or raised plaques, almost always accompanied by fever and a surge of white blood cells called neutrophils that pack into the upper layer of the skin. First described in 1964 by the British dermatologist Robert Douglas Sweet, it is formally known as acute febrile neutrophilic dermatosis. The condition can look alarming, and it sometimes signals something more serious going on beneath the surface, but with the right treatment the skin lesions themselves tend to clear quickly.
What It Looks and Feels Like
The hallmark of Sweet syndrome is the abrupt appearance of tender, bright-red skin lesions. These typically show up on the face, neck, and upper limbs, though they can appear almost anywhere. The lesions are not symmetrical; they tend to cluster on one side or in scattered patches rather than mirroring each other.1Actas Dermo-Sifiliográficas. Sweet Syndrome: A Review and Update They range from small raised bumps (papules) to larger nodules and broad plaques, and they are distinctively painful to the touch rather than merely itchy. The surface can look wet or blistered, and some lesions develop a dusky, purple center.
Fever is nearly universal and often precedes the skin eruption by a day or two. Most people also have a noticeably elevated white blood cell count with a heavy neutrophil predominance. Joint pain, fatigue, and a general feeling of being unwell are common companions. The whole picture tends to come on over a matter of days, which is one reason it can be mistaken for a skin infection.
The Three Settings Where Sweet Syndrome Appears
Clinicians group Sweet syndrome into three categories based on the underlying trigger: classical (also called idiopathic), malignancy-associated, and drug-induced.2PubMed Central. Sweet’s syndrome–a comprehensive review of an acute febrile neutrophilic dermatosis A long-term follow-up study of 93 patients found the breakdown was roughly a third in each of the first two categories, with the remainder split between drug reactions and cases tied to autoimmune or inflammatory conditions.3PubMed Central. Sweet Syndrome: Clinical Presentation, Malignancy Association, Autoinflammatory Disorders and Treatment Response in a Cohort of 93 Patients with Long-term Follow-up
Classical Sweet syndrome is the most straightforward. It often follows an upper respiratory or gastrointestinal infection, and it has a strong female predominance, typically striking women between the ages of 30 and 60. In these cases no underlying cancer or offending medication can be identified, and the episode often resolves on its own or with a short course of treatment, though it can come back.
The Cancer Connection
One of the most clinically important things about Sweet syndrome is its association with malignancy. Roughly one in five patients with Sweet syndrome has an underlying cancer, and in some cohorts the proportion is even higher.4PubMed. Insight into Sweet’s syndrome and associated-malignancy: a review of the current literature The condition can appear before, at the same time as, or after a cancer diagnosis, and a flare of Sweet syndrome in a previously treated cancer patient sometimes signals a recurrence.
Blood cancers dominate the malignancy-associated group. Acute myeloid leukemia (AML) is the single most common culprit, followed by myelodysplastic syndromes.5PubMed Central. Sweet Syndrome: Clinical Presentation, Malignancy Association, Autoinflammatory Disorders and Treatment Response in a Cohort of 93 Patients with Long-term Follow-up In one study specifically looking at AML patients, about 1% developed Sweet syndrome, and over half of those cases were classified as AML with features related to myelodysplasia.6PubMed Central. Characteristics of Sweet’s syndrome in patients with acute myeloid leukemia When researchers compared Sweet syndrome patients with and without malignancy, low blood counts across the board—low white cells, low red cells, and low platelets—were significantly more common in the malignancy group.7PubMed. Characteristics of Sweet syndrome in patients with or without malignancy That pattern makes sense: the underlying blood cancer itself disrupts normal blood cell production.
Sweet syndrome linked to solid tumors is less common but well documented. Among reported solid-tumor cases, cancers of the genitourinary tract are the most frequent, followed by breast and gastrointestinal cancers.8PubMed. Sweet syndrome in patients with solid tumors In solid-tumor cases, tender plaques on the upper extremities and an elevated sedimentation rate appear almost universally, but neutrophilia, which is considered a classic marker, is only present in about 60% of these patients. This is an important difference from classical Sweet syndrome, where a high neutrophil count is the norm, and it means doctors cannot rule out a solid tumor association just because the white count looks unremarkable.
Drugs That Can Trigger It
Drug-induced Sweet syndrome (sometimes abbreviated DISS) is a recognized variant, and the list of potential culprits is longer than most people expect. A large pharmacovigilance analysis found 24 drugs significantly linked to the condition, with cancer drugs and immune-modulating agents topping the list.9PubMed Central. Drug-induced Sweet’s syndrome: pharmacovigilance insights from FAERS with a cross-database consistency assessment in VigiBase via LASSO and multivariable logistic regression The colony-stimulating factors filgrastim and pegfilgrastim, which are commonly given to boost white blood cell production after chemotherapy, carry some of the strongest associations. Azacitidine and decitabine, used to treat myelodysplastic syndromes, are also frequent offenders, as are immunosuppressants like azathioprine and certain biologic agents.
Less intuitively, common antibiotics such as trimethoprim-sulfamethoxazole and even standard anti-inflammatory medications like diclofenac have been implicated.10PubMed Central. Drug-induced Sweet’s syndrome: pharmacovigilance insights from FAERS with a cross-database consistency assessment in VigiBase via LASSO and multivariable logistic regression A French pharmacovigilance database found that antibacterials accounted for about a fifth of suspect drugs and that the median time from starting a medication to developing Sweet syndrome was about two weeks, though the range stretched from a single day to years.11PubMed. Drug-induced Sweet’s syndrome: A case/non-case study in the French pharmacovigilance database The practical takeaway is that Sweet syndrome should be considered in anyone who develops acute painful skin lesions while on a new medication, especially if that medication is a colony-stimulating factor, a hypomethylating agent, or a biologic immunosuppressant.
How Doctors Confirm the Diagnosis
Sweet syndrome is diagnosed through a combination of clinical appearance, blood work, and skin biopsy. The clinical criteria, originally laid out by Su and Liu, look for the characteristic abrupt onset of tender red plaques, fever, neutrophilia, and a rapid response to corticosteroids. Blood work typically shows an elevated white blood cell count with a clear neutrophil predominance, along with a raised inflammatory marker (erythrocyte sedimentation rate or C-reactive protein).12PubMed Central. Diagnosis of Sweet’s syndrome in otolaryngology
The skin biopsy is the cornerstone. Under the microscope, the defining feature is a dense collection of mature neutrophils in the upper dermis, with swelling (edema) between the epidermis and dermis. This pattern is distinctive enough that pathologists can usually pick it out without ambiguity.13PubMed Central. Sweet’s syndrome–a comprehensive review of an acute febrile neutrophilic dermatosis One variant worth knowing about is histiocytoid Sweet syndrome, in which the infiltrating cells look more like histiocytes (a different type of immune cell) than typical neutrophils. This variant can be trickier to diagnose and has its own clinical associations.
Because of the overlap with cancer, a new diagnosis of Sweet syndrome usually prompts age-appropriate cancer screening, and if the blood counts look abnormal, a bone marrow biopsy may follow. The goal is not to alarm the patient but to catch any hematologic malignancy early, since early detection changes outcomes.
Telling It Apart From Look-Alikes
Several other conditions can mimic Sweet syndrome closely enough to cause confusion. Cellulitis and other bacterial skin infections are probably the most common initial misdiagnosis, especially since both present with painful red skin and fever. The key difference is that Sweet syndrome does not respond to antibiotics. Pyoderma gangrenosum, another neutrophilic skin disorder, shares enough features with Sweet syndrome that dermatologists consider the two to be on a spectrum. Classic pyoderma gangrenosum produces ulcers with a distinctive undermined purple border, while Sweet syndrome lesions are raised, non-ulcerated plaques and nodules.14PubMed Central. Neutrophilic dermatoses: pyoderma gangrenosum and Sweet’s syndrome In practice, borderline cases exist where the clinical features straddle both conditions, and biopsy findings are what settle the question.
Erythema nodosum, which also produces painful red nodules, is another common differential. However, erythema nodosum lesions appear almost exclusively on the shins, tend to be deeper in the tissue, and biopsy shows inflammation centered on the fat layer rather than the upper dermis. Vasculitis, drug rashes, and infections like herpes simplex or fungal infections may also be considered, depending on the clinical picture.
Treatment and How Quickly It Works
The good news is that Sweet syndrome generally responds dramatically to corticosteroids. In fact, an excellent response to systemic steroids is built into the diagnostic criteria as a supporting feature. The standard approach is oral prednisone at a moderate dose, and most patients see marked improvement within days.15PubMed Central. Histiocytoid Sweet syndrome recalcitrant to prednisone causing severe scarring One recent study found that every patient treated with corticosteroids showed meaningful clinical improvement, which is a rare batting average in dermatology.
The catch is that steroids come with their own set of problems, especially when used repeatedly. Sweet syndrome has a well-known tendency to recur, and patients who relapse every time steroids are tapered need an alternative strategy. Potassium iodide and colchicine are the leading steroid-sparing options and tend to bring symptoms under control quickly. Dapsone, indomethacin, and cyclosporine have also been used with varying degrees of success, though dapsone and cyclosporine require careful lab monitoring for side effects.16PubMed. Sweet’s syndrome: a review of current treatment options
For truly stubborn cases that fail both corticosteroids and conventional steroid-sparing drugs, newer biologic agents are emerging as a last resort. Anakinra, a medication that blocks the inflammatory signaling molecule interleukin-1, has shown promise in individual patients with chronic, relapsing Sweet syndrome that resisted everything else.17PubMed Central. Refractory Sweet Syndrome Treated with Anakinra Research into the pathways that drive neutrophil recruitment, including inflammasome activation and cytokine release, is pointing toward additional biologic targets.18PubMed Central. Insights Into the Pathogenesis of Sweet’s Syndrome
Recurrence and Long-Term Outlook
For patients with classical Sweet syndrome, the long-term prognosis is generally good. Episodes resolve with treatment, and many people never have another flare. But recurrence is common enough to be a defining frustration of the disease. Some patients experience multiple relapses over years, each requiring a fresh course of therapy. In chronic recurrent cases tied to myelodysplastic syndromes, high-dose prednisone can achieve remission, but the lesions tend to flare again when the dose drops. A study of nine such patients found that thalidomide brought complete skin clearance in most, though side effects including blood clots and confusion limited its long-term use.19JAMA Dermatology. Chronic Recurrent Lymphocytic Sweet Syndrome as a Predictive Marker of Myelodysplasia: A Report of 9 Cases
In malignancy-associated Sweet syndrome, the overall prognosis depends far more on the underlying cancer than on the skin disease itself. Successfully treating the leukemia or myelodysplastic syndrome usually quiets the skin flares as well. When the malignancy is refractory to treatment, Sweet syndrome tends to persist or recur, but even then it rarely becomes life-threatening on its own unless it spreads beyond the skin.
When It Spreads Beyond the Skin
Sweet syndrome is overwhelmingly a skin disease, but in rare cases the same neutrophilic infiltration that drives the skin lesions can show up in other organs. This phenomenon is sometimes called “neutrophilic disease” to reflect its broader scope.20PubMed Central. New Practical Aspects of Sweet Syndrome The lungs, eyes, liver, bones, and central nervous system have all been reported as sites of extracutaneous involvement. Pulmonary Sweet syndrome can mimic pneumonia or a lung mass on imaging, which sometimes leads to unnecessary biopsies before the correct diagnosis clicks into place. Eye involvement, usually in the form of scleritis or conjunctivitis, is another well-known extracutaneous manifestation.
Recognizing that Sweet syndrome can affect organs beyond the skin matters because the treatment is the same (immunosuppression rather than antibiotics or surgery), and a delay in the correct diagnosis can mean unnecessary procedures or inappropriate antibiotic courses. If a patient with known Sweet syndrome develops new symptoms in the lungs, eyes, or joints, the possibility of extracutaneous neutrophilic disease should be on the radar.
Sweet Syndrome in Children
Sweet syndrome is uncommon in children, but it does occur, and the pediatric version has some distinctive features. A literature review of 66 reported pediatric cases found that children under age three were more often male and rarely had an associated malignancy, whereas children over three had an equal sex distribution and a strong link to blood cancers of the myeloid lineage.21PubMed. Pediatric sweet syndrome: case report and literature review In younger children, the most common trigger is a preceding infection.22PubMed Central. Pediatric sweet syndrome
The same review flagged a sobering overall mortality of about 9% in pediatric cases, rising steeply to 40% when cardiovascular involvement was present.23PubMed. Pediatric sweet syndrome: case report and literature review Those numbers are driven primarily by the malignancy-associated cases and by the rare but serious complications of extracutaneous disease. For children with infection-triggered Sweet syndrome and no underlying malignancy, the outlook is much better and the condition generally resolves with standard treatment.
Sweet Syndrome During Pregnancy
Pregnancy-associated Sweet syndrome is rare, accounting for a small fraction of all cases, but it has been described often enough to be recognized as its own subtype. A narrative review identified 33 episodes across 30 patients, with more than half occurring during the second trimester.24PubMed. Sweet syndrome in pregnancy: A narrative review Skin lesions most commonly affected the head and neck, and leukocytosis with neutrophil predominance was found in the vast majority. All cases had the diagnosis confirmed by biopsy.
The reassuring finding from the pregnancy literature is that the condition carries a good prognosis for both mother and baby. There has been no documented increase in infant morbidity or mortality, and the syndrome often resolves on its own after delivery.25PubMed. Treatment of Sweet’s syndrome in pregnancy When treatment is needed because delivery is still weeks away and the symptoms are severe, a short course of oral corticosteroids has been effective without fetal complications. Dapsone has been used in at least one case, and conservative management (watchful waiting) has also been reported, though steroid treatment remains the most commonly chosen approach.26PubMed. Sweet syndrome in pregnancy: A narrative review
Why It Happens at All
The underlying cause of Sweet syndrome remains incompletely understood, but the broad strokes are becoming clearer. The condition involves an exaggerated neutrophilic inflammatory response, driven by a cascade of signaling molecules called cytokines.27PubMed Central. Sweet Syndrome in childhood Something, whether it is an infection, a medication, or a signal from a tumor, trips the immune system into overproducing and mis-directing neutrophils to the skin and occasionally to other organs.
Recent research has shed light on several of the molecular actors involved. Inflammasome activation, a process by which cells detect danger signals and trigger a potent inflammatory response, appears to play a central role. In malignancy-associated cases, there is evidence that the malignant clone itself may undergo transformation into the neutrophils that infiltrate the skin, meaning the “inflammatory” cells are partly cancer-derived.28PubMed Central. Insights Into the Pathogenesis of Sweet’s Syndrome Genetic contributions have also been identified, though no single gene explains the condition. These mechanistic insights are not just academic curiosities; they are the basis for the biologic therapies, like anakinra, now being tested in refractory cases.
One pattern that emerges from the research is that Sweet syndrome sits at a crossroads of the immune system. It overlaps with other neutrophilic dermatoses like pyoderma gangrenosum, shares triggers with autoinflammatory syndromes, and can be provoked by drugs that either boost or suppress immune function. That contradictory pharmacology, where both immunosuppressants and immune-stimulating drugs can trigger the same disease, reflects the complexity of neutrophil regulation. The system can tip into Sweet syndrome from more than one direction, which is part of what makes the condition so clinically variable.