This study is a trial looking at a drug to help prevent acute mountain sickness. The drug is prochlorperazine, a drug commonly used to treat nausea vomiting and headache. Participants will either take the drug or a placebo, and hike to and sleep at the summit of Mount Blue Sky located at 4,348 meters (14,265 feet).

This study will be a double-blind randomized controlled trial to investigate the utility of prochlorperazine maleate versus placebo for the chemoprophylaxis of acute mountain sickness on rapid ascent to 4,348 meters, specifically the summit of Mount Blue Sky in the Mount Blue Sky Wilderness. Participants will be evaluated for AMS utilizing the 2018 LLQ both the evening of and morning after ascent. The primary outcome will be the presence of AMS, defined by a 2018 LLQ score equal to or greater than 3, including the presence of a headache at any measured point during the study. Individuals will convene the morning of the study in Golden, Colorado where they will receive breakfast and either placebo or prochlorperazine. They will then be driven to Summit Lake where they will break for lunch and receive the second dose of placebo or prochlorperazine. They will then hike to the Mount Blue Sky summit where they will receive their third and final dose of either placebo or intervention, spend the night in the structures at the summit, and be assessed for AMS via LLQ. In the morning, they will be assessed for AMS again, receive breakfast, and then be driven back to the original meeting point. They will be monitored by Emergency Physicians at all points during the study.

Inclusion Criteria:

• Adults >18 years old.

Exclusion Criteria:

• Individuals <18 years old or >75 years old
• Pregnant women
• Individuals who reside at or have slept at elevations >1,800 meters in the last two weeks
• Individuals having taken acetazolamide, steroids, ibuprofen, anti-emetics or additional analgesics within the last 24 hours prior to study initiation
• Individuals requiring supplemental baseline oxygen or with chronic disorders known to be significantly impacted by hypoxia
• Individuals with known allergies to prochlorperazine or phenothiazines
• Individuals taking medications with significant medication interactions with prochlorperazine: Dofetilide, potassium acid phosphate, potassium chloride, potassium citrate, potassium phosphate, Yohimbe.
• Individuals with a history of dementia
• Individuals who lack decision making capacity

This study will be a placebo-controlled, double blinded trial. Intervention and placebo are non-identical though very similar in appearance and will be stored in opaque vials. A non-blinded study team member will be solely in charge of dispensing medications.