Can you train to prevent altitude sickness?

Not directly. Arriving with a strong aerobic base helps your body cope with reduced oxygen availability, but it doesn't prevent altitude sickness. The real preparation is proper acclimatisation on the trek itself by ascending slowly and not skipping acclimatisation days.

What are the first signs of altitude sickness?

Headache is usually the first sign, often accompanied by fatigue, nausea, and dizziness. These symptoms appearing at altitude should be taken seriously. If they worsen overnight or after rest, do not ascend further until they improve.

When should you descend for altitude sickness?

If symptoms are worsening despite rest and hydration, descend. Do not sleep at an altitude where you have increasing symptoms. Descent is the most effective treatment for altitude sickness and should not be delayed if symptoms are progressing.

Does Diamox actually work for altitude sickness?

Yes, acetazolamide (Diamox) is the most evidence-backed medication for altitude sickness prevention. It works by stimulating faster breathing and reducing fluid retention. It requires a prescription and is not suitable for people with sulfa allergies.

April 17, 2026 · Nepal prep

Altitude Sickness at Thorong La Pass — What I Actually Need to Know

Until recently my plan for altitude sickness was "I'll deal with it when I get there." I'm aware that's the arrogant approach. I'm also preparing for a 5,416m pass in October, so it's probably time to actually understand what I'm walking into rather than hoping for the best.

A workmate who has done a similar trek told me about it. He was run down, had no energy, had to push through. That was about the extent of the briefing. Not exactly detailed. I've been operating on that level of understanding for months.

I've heard more vivid accounts than that since. One person said they felt like they were about to die. Another person fainted. These are not edge cases people are exaggerating for effect. They are normal accounts from normal people who underestimated what altitude does to a body that has no experience of it.

What I had heard was this: you can't really train for altitude sickness. There are doctors you can see before a trek who can assess you and prescribe medication. When my company's team did an Everest trek, they all saw a medical doctor beforehand. Nobody has recommended that for this trek. But after writing this article, I understand why they did it.

One more thing before we start. Apparently Viagra helps with altitude acclimatisation. I'm not making that up. I'll explain the physiology later and it actually makes complete sense.

What altitude sickness actually is — the physiology

Above a certain elevation, the air still contains 21% oxygen — same percentage as at sea level. But atmospheric pressure is lower. That means each breath delivers fewer oxygen molecules to your lungs. Your body responds immediately: breathing rate increases, breathing deepens, heart rate goes up. It is trying to deliver more oxygen to the tissues that need it.

This is hypoxia. Inadequate oxygen delivery to your tissues. As a physiotherapist I understand the clinical picture — fatigue, headache, confusion, elevated heart rate, breathlessness on minimal exertion. These are the physiological responses to reduced oxygen availability. You can see this in clinical settings with patients who have respiratory conditions. At altitude, you're inducing it by changing elevation.

Altitude sickness, technically Acute Mountain Sickness or AMS, is what happens when your body can't compensate fast enough. The pressure difference also causes fluid to shift within your body. That fluid can accumulate in the brain — this is High Altitude Cerebral Oedema, or HACE. Or it can accumulate in the lungs — High Altitude Pulmonary Oedema, or HAPE. These are the serious versions. They're not common, but they're dangerous and they can progress quickly.

Most people experience something milder than HACE or HAPE. Headache, nausea, fatigue, dizziness, poor sleep, loss of appetite. Unpleasant but manageable if you respond correctly. Thorong La Pass sits at 5,416m. The symptoms become more likely and more pronounced the higher you go. At that elevation, mild AMS is a realistic expectation for many trekkers.

Here is the part that genuinely humbled me: fitness does not protect you. Very fit people get altitude sickness. People in the best shape of their lives have had to turn back from passes lower than Thorong La. Your aerobic fitness helps your body cope with the physical effort, but it does not make you immune to the physiological effects of reduced atmospheric pressure. These are separate systems. I find this both reassuring and unsettling in equal measure.

Why you can't really train for it

This is the thing that surprised me most when I started reading properly. With most physical challenges, you can train specifically for the demand. Running a half-marathon? Run more. Steep terrain? Find hills and hike them with a pack. Altitude? Unless you have access to an altitude tent or you live at elevation, you cannot meaningfully simulate what your body will experience above 4,000m. The stimulus isn't available to you at sea level in Auckland.

What you can do is arrive as aerobically fit as possible. A stronger aerobic base means your body is more efficient at extracting and delivering oxygen to working muscles. When oxygen availability drops, you have more capacity to work with. It doesn't prevent altitude sickness but it gives you a better foundation to manage it. The fit person who gets AMS at Thorong La is in a better position than the unfit person who gets AMS at Thorong La.

The real preparation for altitude is acclimatisation on the trek itself. Ascending slowly, spending nights at lower elevations than your daily high point, giving your body time to produce more red blood cells and adjust to the reduced pressure. This is why the itinerary matters enormously. A well-designed Annapurna Circuit schedule builds acclimatisation days in before Thorong La. Rushing the ascent to save time is how people get into serious trouble. It's not a shortcut. It's a way to guarantee worse symptoms.

The symptoms to actually watch for

Honest admission: I wouldn't have known what to look for before writing this. The "deal with it when I get there" approach is fine for minor inconveniences. It's a bad strategy when the symptom you're ignoring is early HACE. So here is what I now know.

Mild AMS — common, manageable

·Headache — the most common first sign
·Fatigue and weakness beyond what the effort explains
·Dizziness or lightheadedness
·Nausea or loss of appetite
·Poor sleep quality
·General feeling of being run down — this is what my workmate described

Moderate to severe AMS — stop and reassess

·Persistent headache that doesn't respond to paracetamol
·Vomiting
·Increasing fatigue and weakness
·Reduced coordination
·Confusion or unusual behaviour

HACE / HAPE warning signs — descend immediately

·Severe headache, vomiting, confusion
·Inability to walk in a straight line
·Breathlessness at rest
·Persistent cough, especially with pink or frothy sputum
·Extreme fatigue that doesn't improve with rest

The rule of thumb

If symptoms are getting worse, descend. Do not sleep at an altitude where you have worsening symptoms. Descent is the most effective treatment available and should not be delayed.

This is one of the things that actually reassures me about altitude sickness. Unlike a lot of medical situations, the fix is straightforward. You go down. People who have experienced serious AMS consistently report that descending even a few hundred metres brings rapid relief. The mountain is not going anywhere. Getting yourself functional is the priority.

What actually helps

Ascend slowly

The standard guideline above 3,000m is not to increase your sleeping elevation by more than 300 to 500m per day. The Annapurna Circuit, done at a sensible pace, generally follows this. The problem is when people start combining days to save time, or when early symptoms get pushed through because they feel manageable. They often stop feeling manageable.

The climb to Thorong La involves an early morning start from Thorong Phedi or High Camp, typically around 4am, to cross before afternoon winds and weather deteriorate. The altitude gain on that final climb is significant and rapid. This is exactly why the acclimatisation days earlier in the itinerary are not optional extras. They are what makes the crossing possible.

One thing from reading accounts online that genuinely reassured me: the Annapurna Circuit ascends gradually by design. The route follows river valleys upward over multiple days, gaining elevation progressively rather than in one sharp push. This is not accidental. The classic circuit route was developed over decades and the gradual ascent profile is one reason it remains one of the more manageable high-altitude treks despite the elevation. Following the standard itinerary without rushing it is genuinely good acclimatisation strategy, not just good advice.

Hydration

Staying well hydrated helps your body manage at altitude. Breathing rate increases at elevation, which means more fluid loss through respiration even in cool, dry conditions. Dehydration compounds altitude symptoms and makes everything harder to manage.

I built a hiking water calculator with an altitude setting for exactly this reason. At 5,000m in cold dry air, hydration needs are higher than most people expect, even when you're not generating much heat. The calculator accounts for this and gives you a carry estimate adjusted for elevation.

Medication — Diamox and yes, Viagra

Acetazolamide, brand name Diamox, is the standard medication prescribed for altitude sickness prevention. It works by stimulating faster, deeper breathing, which increases oxygen uptake. It also reduces fluid retention. Many experienced high-altitude trekkers use it prophylactically before and during ascent to significant elevation. The standard protocol is to start it 2 to 3 days before ascending to altitude, not just on the day of the climb. This gives the medication time to work before you actually need it. I didn't know that before researching this, and it changes how you'd plan the timing.

It requires a prescription. The pre-trek medical assessment that my company's Everest team went through was primarily about getting this prescription and having baseline health assessed before heading to extreme altitude. I haven't booked anything like that. After writing this article, I think I probably should.

Side effects of Diamox include tingling in the fingers and toes, increased urination, and making carbonated drinks taste flat. Manageable. More importantly, it's a sulfa drug. If you have a sulfa allergy you cannot take it, which is one reason the medical assessment matters — to confirm you're actually a candidate for the medication.

Now — Viagra. Sildenafil works by dilating blood vessels, which reduces pulmonary arterial pressure. At altitude, the lungs respond to low oxygen by constricting blood vessels. This increases the workload on the right side of the heart and is part of the mechanism behind HAPE. Sildenafil counteracts this directly. It has been used in altitude medicine research and is sometimes prescribed for high-altitude pulmonary oedema risk. I'm not planning to take Viagra to Nepal. But I'm genuinely glad I now understand why people do.

Ibuprofen

Some research suggests ibuprofen taken before and during ascent reduces AMS incidence. The evidence is not as strong as for Diamox but it's available without a prescription and the mechanism is plausible — anti-inflammatory effects reducing some of the fluid-related component of symptoms. Worth knowing about as a backup option.

What I'm actually going to do

My updated position after writing this: I'm no longer in the "deal with it" camp. Not because I'm scared. Because I now understand what I would be dealing with and, more importantly, I understand the difference between manageable symptoms and warning signs that require action. That distinction matters a lot at 5,000m.

I'm going to look into a pre-trek medical assessment. Not because it's mandatory but because understanding my baseline and having a Diamox prescription available if needed seems sensible for a 5,416m crossing with zero prior altitude experience. The team that went to Everest all did it. That's a reasonable precedent.

The main preparation remains arriving as aerobically fit as possible and trusting the acclimatisation process on the trek itself. Follow the itinerary, don't rush it, drink more water than feels necessary, and actually pay attention to how I feel rather than defaulting to pushing through everything. That last part is the one I'll need to actively work on.

A note on character and determination

I can see myself wanting to push through discomfort on this trek. That's honestly part of the appeal. I want to know what that kind of effort feels like. But there is a meaningful difference between pushing through the discomfort of sustained physical effort and pushing through warning signs that indicate something your body is not handling.

One of those is determination. The other is dangerous. Knowing the difference is really what this article is about — and the reason writing it was actually useful, not just a content exercise.

Calculate your high altitude water needs →Use the altitude setting in the hiking water calculator to estimate hydration needs above 2,500m.

Frequently asked questions

Can you train to prevent altitude sickness?: Not directly. Arriving with a strong aerobic base helps your body cope with reduced oxygen availability, but it doesn't prevent altitude sickness. The real preparation is proper acclimatisation on the trek itself by ascending slowly and not skipping acclimatisation days.
What are the first signs of altitude sickness?: Headache is usually the first sign, often accompanied by fatigue, nausea, and dizziness. These symptoms at altitude should be taken seriously. If they worsen overnight or after rest, do not ascend further until they improve.
When should you descend for altitude sickness?: If symptoms are worsening despite rest and hydration, descend. Do not sleep at an altitude where you have increasing symptoms. Descent is the most effective treatment for altitude sickness and should not be delayed if symptoms are progressing.
Does Diamox actually work for altitude sickness?: Yes, acetazolamide (Diamox) is the most evidence-backed medication for altitude sickness prevention. It works by stimulating faster breathing and reducing fluid retention. It requires a prescription and is not suitable for people with sulfa allergies.

Matt Jenkinson

Physiotherapist, Auckland NZ

Building HikeCalc to prepare for the Thorong La Pass, Nepal, October 2026.