IL-17 Drugs: How They Work, Differences, and Side Effects

IL-17 drugs are a class of biologic medications that block part of the interleukin-17 signaling pathway, a branch of the immune system heavily involved in driving inflammatory skin and joint diseases. Five are currently approved: secukinumab, ixekizumab, brodalumab, bimekizumab, and (for certain markets) netakimab. They are used primarily for moderate-to-severe plaque psoriasis, psoriatic arthritis, and ankylosing spondylitis, and they have become some of the fastest-acting biologics available for clearing psoriasis. Understanding which drug targets what, how they compare to each other and to other biologic classes, and what risks come with suppressing IL-17 is useful whether you are newly prescribed one or trying to make sense of treatment options with your doctor.

Why IL-17 Matters in Inflammatory Disease

Interleukin-17 is a family of signaling proteins, with IL-17A and IL-17F being the members most relevant to disease. In a healthy immune system, these proteins help fight off bacteria and fungi, particularly at mucosal surfaces like the mouth, gut, and skin. They recruit immune cells to sites of infection and strengthen the barriers that keep pathogens out. The problem arises when the IL-17 pathway becomes overactive without a real infection to justify it. In psoriasis, for example, IL-17A floods the skin and drives the rapid turnover of skin cells that produces thick, scaly plaques. In spondyloarthritis, excess IL-17 contributes to inflammation at the spine and joints.¹

Because IL-17 sits at a critical junction between defense and damage, blocking it can dramatically reduce inflammatory disease activity. But the same pathway that causes psoriatic plaques also protects you from yeast infections, which is why every IL-17 drug carries certain predictable trade-offs.

The Drugs and How They Differ

All five approved IL-17 drugs are injectable monoclonal antibodies, but they do not all hit the same target. The differences in mechanism matter because they influence both efficacy and side-effect profiles.

Secukinumab (Cosentyx): Targets IL-17A specifically. It was the first IL-17 inhibitor to reach the market and has the longest track record, with approvals for psoriasis, psoriatic arthritis, and ankylosing spondylitis.
Ixekizumab (Taltz): Also targets IL-17A. It blocks the interaction between IL-17A and its receptor and has shown potent inhibition of downstream inflammatory signals in cell-based studies.² Approved for the same conditions as secukinumab.
Brodalumab (Siliq): Instead of neutralizing the IL-17 protein itself, brodalumab blocks the IL-17 receptor A subunit. This means it inhibits not just IL-17A but also IL-17F and other family members that signal through the same receptor.³ Approved for psoriasis only.
Bimekizumab (Bimzelx): A dual inhibitor that binds and neutralizes both IL-17A and IL-17F simultaneously. Each binding site on the antibody can grab either protein, making it a single molecule that covers two targets.⁴ Approved for psoriasis and psoriatic arthritis, with additional indications expanding.

The practical distinction between these approaches is that drugs targeting only IL-17A leave IL-17F free to signal through the receptor, while brodalumab and bimekizumab shut down a broader range of IL-17 signaling. Whether this broader blockade translates into meaningfully better outcomes is a question the clinical data has been answering over the past several years.

How Well They Work for Psoriasis

As a class, IL-17 inhibitors are among the most effective treatments for plaque psoriasis, and they tend to work faster than most alternatives. In a comparative analysis of clinical trial data, IL-17 inhibitors reached high levels of skin clearance faster than IL-23 inhibitors. Bimekizumab at 320 mg achieved near-complete clearance (a benchmark called PASI 100) in about five and a half weeks on average, the fastest of any biologic studied. Brodalumab followed at roughly seven weeks, ixekizumab at eight and a half weeks, and secukinumab at ten weeks.⁵

A network meta-analysis that pooled data from across multiple clinical trials reinforced these speed findings. IL-17 inhibitors had the shortest time to reach meaningful clinical improvement, with bimekizumab, brodalumab, and ixekizumab all achieving a 75% reduction in psoriasis severity faster than other biologic classes. No significant difference in speed was found among those three front-runners, though bimekizumab was faster than the remaining biologics overall.⁶

In a head-to-head trial comparing ixekizumab to guselkumab (an IL-23 inhibitor), ixekizumab produced complete skin clearance at 12 weeks in about 41% of patients compared with 25% for guselkumab. Ixekizumab also achieved early milestones like 50% improvement at just one week and 75% improvement at two weeks.⁷ That early speed can matter psychologically. For someone who has lived with visible plaques for years, seeing dramatic improvement in the first couple of weeks of treatment is a different experience than waiting two or three months.

When comparing within the IL-17 class, ixekizumab and secukinumab have been tested against each other in a randomized trial. At 12 weeks, about 77% of ixekizumab patients hit 75% improvement compared with 67% on secukinumab, and the gap in near-complete clearance was wider still. By one year, the differences narrowed but ixekizumab maintained a small edge.⁸ A broader network meta-analysis found that brodalumab, ixekizumab, and the IL-23 inhibitor risankizumab were all significantly more effective than secukinumab at achieving high clearance levels, with the greatest advantage visible at the 90% and 100% improvement thresholds.⁹

Beyond the Skin: Joints and Spine

Psoriasis and psoriatic arthritis frequently overlap, and IL-17 drugs are approved for both. For psoriatic arthritis specifically, the picture is more nuanced than for skin disease. A target trial emulation study found that IL-17 inhibitors and TNF inhibitors performed comparably on joint outcomes after 12 months. Roughly a quarter of patients on either class achieved a 20% improvement in joint symptoms, and remission rates were similar at around 10-13%.¹⁰ There is some evidence that IL-17 inhibitors have an advantage for enthesitis, the painful inflammation where tendons meet bone, with ixekizumab possibly outperforming TNF inhibitors on that particular symptom.¹¹

For ankylosing spondylitis (also called axial spondyloarthritis), IL-17 inhibitors have shown clear benefit. A systematic review and meta-analysis found that at 16 weeks, patients on an IL-17 inhibitor were roughly 63% more likely to reach a meaningful improvement threshold compared with placebo, and about twice as likely to reach the higher improvement bar.¹² Ixekizumab specifically showed improvements in disease activity, physical function, quality of life, and spinal inflammation visible on MRI after just 16 weeks of treatment.¹³ Secukinumab was the first IL-17 inhibitor approved for this condition and remains widely used.

The Candidiasis Trade-Off

The most distinctive side effect of IL-17 drugs is yeast infection, specifically candidiasis. This is not a surprise or an idiosyncrasy of individual drugs. It is a direct consequence of the mechanism. IL-17 signaling is one of the body’s primary defenses against Candida species, so blocking it predictably weakens that defense. A large real-world observational study found that anti-IL-17 therapy was associated with roughly a tenfold increase in candidiasis reports compared with biologic users overall, with the highest risk for oral and esophageal yeast infections.¹⁴

The infections tend to appear within a few months. The same study reported a median onset of about 84 days after starting treatment, much faster than the candidiasis cases seen with TNF inhibitors or IL-12/23 inhibitors. Most of these infections are mild to moderate, treatable with standard antifungal medications, and do not require stopping the IL-17 drug. Serious or invasive candidiasis is rare. Still, if you are prone to recurrent yeast infections or are immunosuppressed from other causes, this is worth discussing with your prescriber before starting treatment.¹⁵

Inflammatory Bowel Disease Concerns

There has been persistent concern about whether IL-17 inhibitors can trigger or worsen inflammatory bowel disease, specifically Crohn’s disease and ulcerative colitis. This concern has a real biological basis: IL-17 signaling appears to play a protective role in the gut, and early clinical trials of secukinumab for Crohn’s disease actually found it made the condition worse. That trial was stopped, and IL-17 drugs are generally avoided in patients with active IBD.

Post-marketing surveillance has flagged reporting signals. An analysis of adverse-event databases found statistically elevated reporting rates for IBD events with secukinumab and ixekizumab, though brodalumab did not trigger the same signal.¹⁶ However, when researchers pooled clinical trial data directly comparing IL-17 inhibitor recipients to placebo, they found no statistically significant increase in new-onset IBD.¹⁷

How to reconcile those two findings? Adverse-event databases capture signals from millions of real-world users, including people with undiagnosed or subclinical gut inflammation who may not have been enrolled in trials. Clinical trials, meanwhile, typically screen out anyone with existing IBD. The practical takeaway is that new-onset IBD on IL-17 therapy is uncommon, but the drugs can unmask or worsen gut inflammation in someone already predisposed. If you have a personal or family history of Crohn’s disease or ulcerative colitis, your doctor will likely steer you toward an IL-23 inhibitor or another class instead.

The Brodalumab Suicide Question

Brodalumab carries a boxed warning about suicidal ideation and behavior, which was added after suicide events occurred in its clinical trial program. This warning has understandably made patients and prescribers cautious. The ongoing pharmacovigilance data, however, has been reassuring. A seven-year post-marketing surveillance report in the United States found no completed suicides throughout the entire monitoring period, and only a single suicide attempt, which occurred in year three.¹⁸ The drug remains available through a restricted distribution program that includes a risk evaluation and mitigation strategy. The evidence so far suggests the actual risk is very low, but the boxed warning persists and limits prescribing uptake in practice.

When IL-17 Drugs Stop Working

Some patients find that an IL-17 drug that worked brilliantly for a year or two gradually loses its punch. This loss of response is often caused by anti-drug antibodies, which are proteins your immune system makes against the therapeutic antibody itself. It is an ironic situation: your immune system treats the drug that is calming your immune system as a foreign invader. These antibodies can neutralize the drug in your bloodstream before it reaches its target, effectively lowering your dose without anyone changing it.¹⁹

The good news is that IL-17 inhibitors appear more resilient to this problem than some older biologics. An analysis of existing evidence found that while anti-drug antibodies against TNF inhibitors and IL-12/23 inhibitors were clearly linked to lower response rates, only high antibody levels reduced the clinical effects of IL-17 inhibitors. Low or transient antibody levels did not seem to matter much.²⁰ This relative resistance to immunogenicity is one of the advantages of the IL-17 class, though it does not eliminate the problem entirely. When a patient does lose response, switching to a different mechanism within or outside the IL-17 class is the standard approach.

Cardiovascular Side Benefits

Psoriasis is associated with elevated cardiovascular risk, likely because the same systemic inflammation that drives skin disease also accelerates atherosclerosis. There has been growing interest in whether biologics that control that inflammation might also lower heart disease risk. A large population-based cohort study found that psoriasis patients receiving IL-17 inhibitors had a roughly 28% reduction in the risk of cardiovascular events compared with those not on biologics. IL-23 inhibitors showed a similar reduction, while TNF inhibitors had a more modest benefit. IL-12/23 inhibitors did not reach statistical significance.²¹

This is observational data, not a randomized trial, so there are confounders to consider. Patients prescribed newer biologics like IL-17 and IL-23 inhibitors may differ in important ways from those on older drugs. Still, the finding is consistent with the idea that effectively controlling inflammation has downstream cardiovascular benefits, and it adds another consideration to treatment selection for patients with psoriasis who also carry heart disease risk factors.

Use in Children and Adolescents

Secukinumab and ixekizumab are approved for pediatric psoriasis, and the evidence in children largely mirrors what has been seen in adults. A real-world multicenter study across multiple countries confirmed that IL-17A inhibitors are highly effective in pediatric psoriasis with a favorable safety profile.²² Their efficacy appears comparable to what is seen with TNF inhibitors in the same age group, and the safety profile is similar as well.²³ Brodalumab and bimekizumab are not yet approved for children, though studies are ongoing.

Pediatric psoriasis is more than a skin inconvenience. It can severely affect quality of life, school attendance, and social development, and historically treatment options for children were limited to topical therapies and older systemics with more side effects. Having IL-17 drugs available for younger patients has meaningfully expanded the toolkit, particularly for severe cases that do not respond to first-line treatments.

Emerging Conditions Under Study

Researchers are testing IL-17 drugs in conditions beyond their current approvals. Bimekizumab has shown promise for hidradenitis suppurativa, a painful chronic skin condition involving inflamed nodules and tunnels under the skin. A phase 2 trial found that dual inhibition of IL-17A and IL-17F may be a viable treatment approach for hidradenitis, with the potential for deep clinical responses.²⁴ This is early-stage evidence, but hidradenitis suppurativa is notoriously difficult to treat and any new effective option would be significant.

Oral IL-17 Inhibitors on the Horizon

Every currently approved IL-17 drug requires injection, either self-administered at home with a prefilled pen or syringe, or sometimes given in a clinic. For some patients, the injections themselves are a barrier. Needle anxiety, injection-site reactions, or simply the inconvenience of cold-chain storage can limit adherence. Several pharmaceutical companies are developing oral small-molecule inhibitors that block IL-17A without the need for injections.

A first-in-human study of one such compound, LY3509754, assessed safety and target engagement in healthy volunteers. The drug was well tolerated at the doses tested and showed evidence of engaging IL-17A in the bloodstream.²⁵ Other research groups have identified novel classes of small molecules that bind to IL-17A in a previously undescribed way, fitting symmetrically into cavities in the protein and blocking it from interacting with its receptor.²⁶

These are still in early development. It typically takes many years for a compound that works in phase 1 to reach pharmacy shelves, and many never make it. But the trajectory is clear: the field is moving toward a future where IL-17 inhibition might be available as a daily pill rather than a biweekly or monthly injection. If the oral versions match even a fraction of the efficacy of the injected antibodies, they could reshape treatment patterns for millions of people with psoriasis and related conditions.

Cost and the Economics of Access

Biologic drugs are expensive, and IL-17 inhibitors are no exception. In the United States, list prices for these medications typically run into tens of thousands of dollars annually, though actual out-of-pocket costs vary enormously depending on insurance, copay assistance programs, and whether biosimilars are available. Secukinumab, the oldest IL-17 inhibitor, is the first in the class to face potential biosimilar competition, which may eventually lower costs.

Health-economic analyses try to determine which drugs give the best value for money. A cost-effectiveness study comparing bimekizumab to other IL-17 inhibitors for axial spondyloarthritis in Scotland found that bimekizumab had a modest cost-effectiveness advantage over secukinumab, while bimekizumab and ixekizumab had similar overall costs and health outcomes.²⁷ These comparisons are highly context-dependent, varying by country, healthcare system, and the specific prices negotiated by insurers. For patients in the United States, manufacturer copay programs can reduce costs substantially for those with commercial insurance, but access remains more complicated for uninsured or underinsured patients. Working with a specialty pharmacy and exploring patient assistance programs early in the process can make a real difference.