A Third of Rare Disease Trials Can Shut You Out Over a Medication You Already Take

Somebody with myasthenia gravis who is doing well on Vyvgart can find a promising new trial, read to the bottom of its eligibility list, and learn that the drug keeping their symptoms in check is the one thing keeping them out. That is a common outcome. Of the 85 open myasthenia gravis treatment trials we checked, 21 (25%) exclude people taking efgartigimod or the FcRn blocker class it belongs to.

We wanted to know how often this happens across rare disease, so we measured it. On September 28, 2026, we pulled every recruiting or not-yet-recruiting treatment trial on ClinicalTrials.gov for the 37 conditions that have a medication checker on Trial Friend, 1,853 unique trials in all, and ran each trial's exclusion criteria through the same checker patients use on our disease pages.

Which Rare Disease Clinical Trials Exclude the Most Medications

The share swings widely by disease. Nearly 2 in 3 myasthenia gravis trials (65%) name a commonly used medication in their exclusions, and so do more than half of IgA nephropathy (55%) and Cushing disease (52%) trials. At the low end sit ALS, where 15% of trials do, and pulmonary arterial hypertension, at 16%.

Immune diseases such as myasthenia gravis and IgA nephropathy lead the list. When most patients already take a steroid, an immunosuppressant or one of a few newer biologic drugs, a trial of the next immune drug often wants people off the old one first, either to keep the two from stacking up or to see what the new drug does on its own. ALS and pulmonary arterial hypertension sit much lower. Some ALS trials go the other way entirely: in our data, 2 exclude people who will not take riluzole, and another asks only that riluzole and edaravone doses have been stable for 1 to 2 months before enrolling.

Sorting every match into 5 kinds of medicine shows where the exclusions come from. Steroids such as prednisone and dexamethasone appear in the exclusion rules of 41% of IgA nephropathy trials, 39% of Cushing disease trials, 33% of bile duct cancer trials and 30% of idiopathic pulmonary fibrosis trials. Immune-targeting drugs (immunosuppressants, complement blockers, FcRn blockers and B-cell drugs) lead in myasthenia gravis, at 58%.

Steroid rules usually come with limits attached. Many set a ceiling, such as more than 10 mg of prednisone a day, or a window, such as no steroid course in the last 4 weeks. Someone on a small maintenance dose may still qualify, and that is exactly the kind of detail to raise with the study coordinator before assuming a trial is closed to you.

Gene Therapy Is the Clinical Trial Exclusion You Cannot Wait Out

A washout works because a drug eventually clears your body. A one-time gene therapy is built to keep working for years, so there is nothing to wait out, and a trial that excludes anyone who has had one excludes them for good. Among diseases with an approved gene therapy in our checker, that rule shows up in 27% of thalassemia trials, 26% of retinitis pigmentosa and Duchenne trials, 18% of spinal muscular atrophy trials and 11% of sickle cell trials.

None of this makes gene therapy the wrong choice. It means the order can matter, so if you are weighing a one-time treatment and a trial at the same time, ask your specialist to look at the open trials before the infusion rather than after. Huntington disease trials show the same pattern for investigational gene-targeting treatments such as AMT-130, with 32% excluding people who have already had one.

How Long Is a Washout Period Before a Clinical Trial

Of the exclusion rules we found, 379 name a specific time window, such as within 4 weeks or within 6 months before the first dose, and some give different windows for different drugs, for 410 windows in all. The median window is about 2 months (61 days). Roughly 45% are 1 month or shorter, yet 48% reach back about 3 months (12 weeks) or more, and 23% reach back about 6 months (24 weeks) or longer.

The longest windows belong to drugs that stay active in the body for months. B-cell drugs such as rituximab and ocrelizumab have a median window of 6 months across the 52 windows that name them, and complement and FcRn blockers sit at a median of about 3 months across 38. Steroid windows are much shorter, with a median of 30 days across 137.

Regulators have started questioning windows set by habit. In July 2026 the FDA finalized guidance for cancer trials saying time-based washout periods should be scientifically justified using what is known about how the earlier drug behaves in the body, and that people taking other medicines should only be excluded when a known or predicted drug interaction or overlapping side effect would affect their safety. The guidance is written for cancer drugs, though the FDA notes many of its concepts may apply to other areas of medicine.

What to Do If Your Medication Could Exclude You From a Clinical Trial

Read the whole rule

Look for a dose, a time window or a stable-dose requirement. A rule such as no more than 10 mg of prednisone a day, or no rituximab in the last 6 months, may already fit your situation.

Ask before you change anything

The study coordinator can tell you whether a rule applies to you and whether the trial expects a washout. Stopping a medicine on your own can bring back symptoms or cause withdrawal effects, so never stop one just to qualify.

Bring in the doctor who prescribes it

If a washout is needed, that doctor has to agree it is safe and plan what happens if symptoms return during the break.

Check every open trial at once

The medication checker on each disease page runs your medicines against every open trial for that condition and quotes the trial's exact wording, so you can see which trials are affected before you call anyone.

Think about the order of one-time treatments

For a gene therapy or another treatment that cannot be undone, look at open trials before treatment rather than after, since many trials exclude people who have already had one.

How We Measured Medication Exclusions in Rare Disease Trials

The data comes from the ClinicalTrials.gov public data API, queried on September 28, 2026 with the same condition searches and exclusion lists our disease pages use. We kept interventional (treatment) trials that were recruiting or not yet recruiting for the 37 conditions that have a Trial Friend medication checker, and counted each trial once even when it appears under 2 related conditions.

A trial counted as excluding a medication when its exclusion criteria named a drug from that condition's checker, or named that drug's class, such as FcRn blockers or anti-CD20 antibodies. Broad wording like any investigational drug, and blanket rules like any cancer treatment in the last 4 weeks, were not counted. Lines that only concern allergies to the study drug, pregnancy, or a stated exception (for example, anti-PD-1 drugs other than pembrolizumab) were not counted either.

We checked the checker by hand. In a random sample of 100 matches, 63 were clear exclusions of people taking that medication, 23 applied only to some users (a dose limit, a recent start or a stable-dose rule) and 14 were wrong. In a random sample of 40 flagged trials, 34 had at least 1 correct match. We also read 30 trials the checker had not flagged and found 6 medication rules it missed; 2 were blanket cancer-treatment rules we leave out on purpose, and we fixed the wording gaps behind the other 4 before the final count. The errors run in both directions, so treat the 33% as a close estimate rather than an exact figure. Time windows were only read when they sat in the same sentence as the medication, and a hand check of 25 found 22 correct. In the time window figures, windows of 11 to 13 weeks count as about 3 months and 23 to 26 weeks as about 6 months, since trials often write 12 or 24 weeks where others write 3 or 6 months.

Both files are free to download and reuse under a CC BY 4.0 license on our open data page: one summarizes each of the 37 conditions and the other lists all 2,162 matches with each trial's exact exclusion wording, time window and ClinicalTrials.gov link. Please cite it as Trial Friend, Medication Exclusions in Rare Disease Trials (2026-09-28), trialfriend.com/data.