Drinking 99.99% hydrogen peroxide is a medical emergency and can cause severe burns, vomiting, internal tissue damage, oxygen gas embolism, breathing failure, and death. Concentrated hydrogen peroxide is far more dangerous than household 3% peroxide, and poison experts warn not to make yourself vomit; emergency help and poison control are recommended right away.

What it can do

  • Burn the mouth, throat, esophagus, and stomach.
  • Release oxygen gas inside the body, which can cause dangerous bloating or gas bubbles in the bloodstream.
  • Trigger chest pain, trouble breathing, confusion, seizures, or collapse.
  • Cause delayed lung injury, sometimes not obvious for up to 24 hours after exposure.

What to do now

  • Call emergency services right away if someone swallowed it and has symptoms.
  • Contact Poison Control immediately.
  • Do not induce vomiting unless a clinician tells you to.
  • If there was skin or eye contact, rinse with lots of water right away.

Why this is so dangerous

Hydrogen peroxide breaks down into water and oxygen, and very concentrated solutions can generate enough gas to injure tissue or block blood flow. Medical sources describe concentrated ingestion as capable of causing caustic injury, gas embolism, and serious gastrointestinal damage.

If this question is about a real exposure, this needs urgent medical care now.