Tirzepatide Plus a Psoriasis Biologic: What the Trial Found

In July 2026, JAMA Dermatology published the results of TOGETHER-PsO, the first phase 3 randomized trial to treat plaque psoriasis and obesity at the same time and measure whether both improved. Everyone in the trial took ixekizumab, a biologic already approved for plaque psoriasis. Half of them also took tirzepatide. At week 36, the combination group had substantially more weight loss and somewhat more complete skin clearance than the group on the biologic alone.

That is one trial, it was funded by the company that makes both drugs, and nobody taking them was blinded. Those things matter, and we will come back to them. But the question it asked has been sitting unanswered for a long time.

Why anyone tried this

Two observations set it up.

First, biologics for psoriasis are given at a fixed dose regardless of how much you weigh. A larger body distributes the same amount of drug across more tissue.

Second, excess adipose tissue is not inert padding. It is metabolically active and contributes to the inflammatory signaling that drives plaque psoriasis. On that reasoning, carrying more of it means a higher baseline inflammatory load for a drug to overcome.

The idea that this actually changes outcomes is older than GLP-1 medications. In a 2008 randomized, investigator-blinded trial published in the American Journal of Clinical Nutrition, 61 obese patients with moderate-to-severe plaque psoriasis all received low-dose cyclosporine, and half also followed a calorie-controlled diet. After 24 weeks, 66.7% of the diet group reached PASI 75 (a 75% reduction in psoriasis severity score) versus 29.0% of those on the drug alone. Losing weight made the psoriasis drug work better in those participants. What was missing until recently was a medication that produces that much weight loss reliably enough to test alongside a modern biologic.

What TOGETHER-PsO tested, and what it found

Eli Lilly, which makes both ixekizumab and tirzepatide, sponsored the trial and randomized 274 adults across 72 US sites. Participants had moderate-to-severe plaque psoriasis (mean PASI 19.7, mean 26.4% of body surface involved, an average of 14.6 years with the disease) and a mean BMI of 39.2. About a third had already tried an advanced therapy. All received ixekizumab; 138 also received tirzepatide. The trial was open-label, meaning participants and their doctors knew which group they were in, although the assessors scoring the skin were kept masked.

The pre-specified primary endpoint was deliberately hard: reaching complete skin clearance and at least 10% weight reduction at the same time, at week 36. In the combination group, 27.1% of participants did. In the biologic-alone group, 5.8% did (P < .001).

Broken out:

• At least 10% weight reduction: 69.2% of the combination group, versus 9.1% of those on the biologic alone (P < .001). Mean weight change was -14.6% versus -1.2%.
• Complete skin clearance (PASI 100): 40.6% versus 29.0% (P = .04).
• PASI 90 and PASI 75: no statistically significant difference (75.6% vs 65.1%, and 91.5% vs 87.2%).
• Blood pressure, hemoglobin A1c, triglycerides, and total cholesterol: all improved more in the combination group.

Read that third line carefully, because it is the honest shape of the result. The biologic on its own already brought most participants to 75% and 90% clearance. The extra skin benefit appeared only at total clearance, and the confidence interval around it ran from 0.3% to 22.9%, meaning the true size of that edge could be close to nothing. The weight-loss difference is large and unambiguous. The skin difference is real but modest and imprecisely measured.

Side effects followed what you would expect from adding tirzepatide. Nausea occurred in 24.6% of the combination group versus 2.2% of the biologic-alone group, diarrhea in 14.5% versus 1.5%, and constipation in 13.0% versus none. Most events in both arms were mild to moderate, and gastrointestinal ones generally eased over time. Discontinuation because of an adverse event was 3.6% versus 1.5%. Serious adverse events were somewhat less common in the combination arm (3.6% versus 6.0%), and there were no deaths.

A second study pointing the same way

A much smaller study published July 30, 2026 in RMD Open looked at psoriatic arthritis rather than skin psoriasis. Italian rheumatology centers followed 31 patients with obesity and psoriatic arthritis who started tirzepatide at the same time as a biologic, matched 1:2 against 62 patients who started a biologic alone. At 6 months, the tirzepatide group had greater BMI reduction (-2.9 vs +0.4 kg/m2), lower C-reactive protein (4.2 vs 7.7 mg/L), and higher rates of an acceptable symptom state on the PsAID-12 questionnaire (48% vs 21%) and of near-clear skin (74% vs 52%).

This was not a randomized trial. The 31 tirzepatide patients were enrolled prospectively while their comparison group came from records going back to 2021, and 31 people is a very small sample. It corroborates the direction of the finding. It does not confirm the size.

What this means if psoriasis and weight are both in your picture

A few things are worth being plain about.

Tirzepatide is approved for chronic weight management and type 2 diabetes, not for psoriasis. As of August 2026, no regulator has evaluated the ixekizumab-plus-tirzepatide combination for treating psoriasis, and using it with that intent is off-label. These results are the start of a case, not an approval.

The trial also says nothing about people whose psoriasis has already failed an IL-17 inhibitor. They were excluded, and a separate trial is studying them. And because there was no tirzepatide-only arm, the trial cannot separate how much of the skin benefit came from weight loss itself versus some direct effect of the drug.

The clearest finding here is about weight, not skin. If you have moderate-to-severe psoriasis along with excess weight, this trial supports treating the weight as part of the same picture rather than as a separate problem. It does not show that adding a GLP-1 or GIP medication will clear skin that a biologic could not.

If that describes you, the conversation belongs with the provider managing your psoriasis, and it starts with an accurate read on where your current treatment actually stands.