Suing in district court to invalidate a patent costs $3-5M and takes years. The USPTO's post-grant review system invalidates patents for $200-500k and takes 12-18 months. For a defendant facing a weak patent, the choice is usually obvious.
There are three flavors. Knowing which to use, when, is the difference between killing the patent and losing the right to challenge it later.
Inter Partes Review (IPR)
The big one. Created by the America Invents Act in 2011 and live since 2012, IPR lets any party (except the patent owner) challenge a patent's claims as anticipated or obvious under § 102 / § 103, based on prior art consisting of patents and printed publications only. § 101 (subject matter) and § 112 (written description) are not available in IPR.
Key facts:
- Cost. USPTO filing fee is in the tens of thousands; total cost typically $200,000–$500,000 including attorney fees.
- Petitioner's burden. Preponderance of the evidence — lower than the "clear and convincing" standard in district court, which is the main reason IPRs invalidate patents at the rates they do.
- Timeline. 6 months from petition to institution decision; 12 months from institution to final written decision. Total: 18 months max.
- Outcome rates. Of instituted IPRs, the majority result in at least one claim being found unpatentable. Many find every challenged claim unpatentable. The current numbers are published quarterly in the USPTO PTAB Trial Statistics.
- Estoppel. This is the critical catch. If you file an IPR and lose, you cannot later raise in district court any prior-art ground you raised "or reasonably could have raised." File carefully — your strongest art has to be in the petition.
When to use IPR:
- You're sued and the patent has prior-art weaknesses. (Prior art and § 102 →)
- You're at risk of being sued and want to clear the air.
- You're a competitor who wants to deny the patentee leverage.
When NOT to use IPR:
- Your strongest defense is § 101 (Alice). IPR can't reach § 101; stay in district court.
- Your strongest defense is non-infringement, not invalidity. The PTAB doesn't decide infringement.
- The estoppel risk outweighs the upside — you'd be foreclosing future arguments to take a 50/50 shot.
Post-Grant Review (PGR)
PGR is broader than IPR but has a tight window. Available only within 9 months of patent issuance, PGR allows challenges on any patentability ground — § 101, § 102, § 103, § 112. Prior art isn't limited to patents and publications.
Most patents you'd want to challenge are older than 9 months by the time anyone notices, so PGR is rarely used in practice. When it does apply — a competitor just got a patent that threatens your business — PGR is more powerful than IPR.
- Cost and timeline are similar to IPR.
- Estoppel is broader than IPR (covers any ground that "was raised or reasonably could have been raised").
Ex parte reexamination
The original post-grant tool, predating IPR. Anyone can file. The proceeding is between the patent owner and the USPTO — the requester drops out after filing. No estoppel. Cheaper than IPR or PGR.
Pros. Cheap, no estoppel, can be filed anonymously.
Cons. The requester has no input after filing. The patent owner can amend claims and the examiner negotiates with them alone. Outcomes are noticeably more favorable to patent owners than IPR.
When to use ex parte reexamination:
- The prior art is overwhelmingly damaging — strong enough that the examiner alone will recognize it.
- You want anonymity (the requester can be anonymous).
- IPR estoppel risk is unacceptable and PGR isn't available.
Combining with district-court litigation
Defendants commonly file IPR while a parallel district-court infringement case proceeds. The district court often stays the litigation pending IPR, especially after institution — saving the defendant millions in discovery and trial costs. Whether stay is granted depends on the judge and the timing.
If the IPR succeeds, the litigation typically dies. If it fails, the district-court case resumes — but the defendant has burned the IPR estoppel and lost cheap invalidity grounds.
Why the system actually works
Across the IPR program's history, the majority of instituted petitions result in at least one claim being unpatentable. A substantial fraction result in all challenged claims being unpatentable. The PTAB's pro-patentee tilt has shifted over the years, but the system remains far more invalidity-friendly than district court.
Translation: when bad patents face real scrutiny at the USPTO, they overwhelmingly lose. The trolls' leverage isn't the patents themselves — it's the cost of getting them in front of the PTAB at all. (What is a patent troll? →)
Bottom line
For a defendant with prior-art-based defenses, IPR is usually the most cost-effective way to kill a patent. The estoppel rules are the trap: file your best art, file it well, and don't file at all if your strongest grounds aren't patents and publications. (See an example dossier with a § 102 / § 103 analysis — the kind of pre-built record that makes an IPR petition cheaper to assemble.)
This article is for general education and is not legal advice.