Pityriasis rosea is most likely caused by the reactivation of common human herpesviruses, specifically HHV-6 and HHV-7, that nearly everyone carries from childhood. It is not caused by bacteria, fungi, or allergies, and it is not considered contagious. The rash typically lasts six to eight weeks and resolves on its own without treatment.
The Viral Reactivation Theory
For years, pityriasis rosea was labeled “cause unknown.” That picture has shifted. A growing body of evidence points to human herpesvirus 6 (HHV-6) and human herpesvirus 7 (HHV-7) as the most likely triggers. These are not the viruses that cause cold sores or genital herpes. HHV-6 and HHV-7 are extremely common viruses that infect most people during infancy or early childhood, often without symptoms, and then go dormant in the body permanently.
The key word is “reactivation.” In a 2024 study published in Diagnostics, researchers found HHV-6 DNA in the blood of 40% of pityriasis rosea patients and in the skin tissue of 24%. HHV-7 showed active infection markers as well, with DNA detected in 16% of tissue samples. Among the patients studied, 16% had both viruses active simultaneously. None of the healthy control subjects showed these same markers. The pattern of antibodies and viral DNA in blood and skin samples is consistent with the viruses “waking up” rather than a new infection.
What triggers this reactivation isn’t fully understood. It may be stress, illness, immune suppression, or seasonal factors. Pityriasis rosea occurs more frequently in spring and fall, which fits the pattern of other viral reactivation conditions. The rash itself appears to be the body’s immune response to the virus becoming active again, not direct viral damage to the skin.
Medications That Can Trigger It
Some drugs cause a rash that looks identical to pityriasis rosea. This is sometimes called a “pityriasis rosea-like drug eruption,” and it can be difficult to distinguish from the viral form. Common culprits include ACE inhibitors (used for blood pressure), hydrochlorothiazide (a diuretic), barbiturates, metronidazole (an antibiotic), allopurinol (used for gout), and certain anti-inflammatory drugs like nimesulide. The drug-induced version tends to have a few differences: the rash may be more widespread, last longer, and sometimes lacks the characteristic first patch that typically kicks off the classic form. If you developed the rash shortly after starting a new medication, that connection is worth raising with your doctor.
What the Rash Looks Like
Pityriasis rosea follows a distinctive two-stage pattern. It usually begins with a single oval patch, often called the “herald patch,” that appears on the chest, back, or abdomen. This patch is larger than the ones that follow, typically two to four centimeters across, with a slightly raised border and fine scaling in the center.
Within one to two weeks, a secondary eruption of smaller oval patches spreads across the trunk. These lesions follow the natural tension lines of the skin, creating a pattern often described as resembling a Christmas tree when viewed from behind. The secondary rash can continue appearing for up to six weeks after the herald patch. The arms and legs may be involved, but the rash rarely appears on the face, hands, or feet. About 50% of people experience mild to moderate itching, while some have no itch at all.
How Long It Lasts
The rash resolves on its own in the vast majority of cases. Most people see clearing within five weeks, and over 80% are fully clear by eight weeks. The mean duration is about 45 days, though in some cases it can stretch to 12 weeks. No scarring occurs, though temporary changes in skin color (lighter or darker patches where the rash was) can linger for a few months, particularly in people with darker skin tones. Recurrence is uncommon, happening in roughly 2% to 3% of cases.
Treatment Options
Because the rash resolves without intervention, treatment focuses on comfort. For mild itching, over-the-counter antihistamines and moisturizing lotions are usually enough. Calamine lotion or low-strength topical steroid creams can help with more persistent itch.
For people with severe or widespread rashes, antiviral medication has shown promise. In a clinical trial, patients who took an antiviral for seven days saw their lesions clear in an average of 18.5 days, compared to 37.9 days in the placebo group. By day 14, 79% of treated patients had full regression of their rash versus just 4% of untreated patients. This supports the viral reactivation theory and gives an option for people whose rash is especially uncomfortable or cosmetically distressing. UV light therapy has also been used for stubborn cases, particularly to reduce itching.
Why It Gets Confused With Other Conditions
Pityriasis rosea is sometimes mistaken for ringworm, eczema, or psoriasis, especially early on when only the herald patch is visible. The more serious concern is secondary syphilis, which can produce a rash that looks remarkably similar. Syphilis rashes tend to involve the palms and soles (which pityriasis rosea almost never does), and a simple blood test can rule it out. Doctors will sometimes order this test as a precaution, particularly in sexually active adults. A skin biopsy can also distinguish the two conditions based on the types of immune cells present in the tissue.
Pityriasis Rosea During Pregnancy
Earlier reports raised concern that pityriasis rosea in early pregnancy might increase the risk of miscarriage, particularly when the rash appeared before 15 weeks of gestation. More recent research has challenged this. A retrospective study using matched controls found no increased risk of miscarriage or birth complications in pregnant patients with pityriasis rosea compared to the general population. This held true even for women diagnosed before 15 weeks. While any new rash during pregnancy warrants a medical evaluation, the current evidence suggests pityriasis rosea does not pose a specific threat to pregnancy outcomes.