Almost every Peru itinerary contains the same design flaw. You land in Lima at sea level, connect the next morning, and ninety minutes later you are standing in Cusco at 3,399 metres carrying your own bags up a cobbled hill. Published clinical guidance puts the rate of altitude illness in exactly this scenario — rapid air ascent to cities above 3,350 m — at roughly 30 to 40 per cent.
That is not a warning against going. It is an argument for building the first two days differently, because the fix costs one transfer and no money at all.
The number nobody expects: Machu Picchu is the low point
Travellers plan around altitude at Machu Picchu and are then blindsided by Cusco. It is the wrong way round.
Machu Picchu sits at 2,430 m. Cusco sits at 3,399 m. The ruins are roughly 970 metres lower than the city you fly into, and most people feel noticeably better there than they did on their first morning in Cusco. The hard part of a Machu Picchu day is the stone stairs, not the air.
The same logic applies across the region. The Sacred Valley floor — Urubamba, Ollantaytambo — sits several hundred metres below Cusco. Rainbow Mountain, by contrast, tops out above 5,000 m, which is 1,600 m higher than the city, and it is regularly sold as a day trip to people who arrived 48 hours earlier.
What altitude illness actually is
Acute mountain sickness is not exotic. According to the US Centers for Disease Control and Prevention, it typically appears 2 to 12 hours after arriving at height and presents much like a hangover: headache is the cardinal symptom, usually with at least one of loss of appetite, dizziness, fatigue, nausea or vomiting.
It can begin at sleeping elevations of about 2,450 m and above. Cusco is comfortably past that threshold. At around 3,000 m, arterial oxygen saturation in an unacclimatised person falls to roughly 88–91 per cent.
The reassuring part: symptoms usually resolve within 12 to 48 hours if you stay at the same elevation and do not climb higher. The dangerous part is ignoring worsening symptoms and going up anyway.
The symptoms that are not ordinary AMS
Two rarer conditions — fluid on the lungs and swelling of the brain — are medical emergencies rather than discomforts. Breathlessness at rest, a wet cough, confusion, or an inability to walk a straight line are not part of a normal adjustment. The response to those is descent, immediately, and medical help. Do not wait to see whether the morning is better.
How fast you can safely go up
The Wilderness Medical Society guidance summarised by the CDC is specific, and it is worth reading against your actual itinerary:
- Avoid going straight to a sleeping elevation above about 2,750 m in a single day.
- Once above roughly 3,000 m, raise your sleeping elevation by no more than about 500 m per night.
- Add an extra rest day for every 1,000 m of elevation gained.
Now hold a standard Peru itinerary against that: Lima at 150 m to Cusco at 3,399 m, in one morning. It breaks all three rules at once. Almost everyone does it anyway, which is precisely why the illness rate is what it is.
Note that these are rules about sleeping elevation. Going high during the day and coming down to sleep is a different and much gentler proposition.
The Sacred Valley strategy
The single most effective change you can make is also the least known: do not sleep in Cusco on your first night.
The CDC notes that where it is feasible, spending one or two nights at a lower elevation before ascending reduces the need for medication. In Peru this is unusually easy to arrange. Transfer straight from Cusco airport down to the Sacred Valley — Urubamba or Ollantaytambo, several hundred metres lower — sleep there for two nights, see Pisac and Ollantaytambo while you adjust, and come back up to Cusco afterwards.
You lose nothing. The Sacred Valley is on the itinerary anyway, and it is on the train line to Machu Picchu. You simply do it first instead of second. The cost is one airport transfer.
If you must sleep in Cusco on night one, at least choose accommodation that is flat to reach. A room up forty steps in San Blas is a different experience on day one than on day four.
What helps, what does not, and what is just hospitality
Time is the only thing that genuinely acclimatises you. The acute phase of adjustment happens over the first three to five days, during which your breathing rate and oxygenation improve measurably.
Water and rest. Unglamorous and effective. Plan nothing for the afternoon you arrive: not a walk to Sacsayhuamán, not a market, nothing.
Alcohol in the first 48 hours is a poor idea; its effects are amplified and it disrupts the broken sleep you are already going to have.
Coca tea will be handed to you before you have taken your coat off, in every hotel in the highlands. It is a genuine local custom, mildly stimulating, and pleasant. It is not a treatment for altitude illness, and it will not let you skip the rest day.
Oxygen canisters sold in Cusco pharmacies provide brief relief and no acclimatisation. Hotels can usually arrange proper oxygen if someone is genuinely unwell.
Fitness does not protect you. Marathon runners get altitude sickness. It correlates with ascent rate and individual physiology, not with how strong you are, and maximal exercise capacity is reduced at altitude even after you have fully adjusted.
Medication: a conversation for your doctor, not a shopping trip
Acetazolamide is the drug most commonly used to speed acclimatisation; the CDC describes a preventive regimen of 125 mg every 12 hours, started the day before ascent. Dexamethasone is used for moderate to severe symptoms and carries a rebound risk if stopped too early.
Both are prescription decisions that depend on your medical history, other medications and allergies. Given that flying into Cusco puts you in the high-risk category, the CDC explicitly advises a low threshold for considering preventive medication in this situation, which is a reason to raise it with your doctor before you travel, not to buy something over a pharmacy counter in Cusco because a stranger recommended it.
Building the first four days
A schedule that respects the physiology and loses nothing worth having:
- Day 1: fly into Cusco, transfer straight down to the Sacred Valley. Do nothing else.
- Day 2: Ollantaytambo or Pisac at valley level. Walk, do not climb.
- Day 3: train to Aguas Calientes, sleep there. Lower again.
- Day 4: early entry at Machu Picchu, at 2,430 m, by which point you have had three nights to adjust and are at the lowest point of the trip.
Cusco itself comes afterwards, when the city is a pleasure rather than a struggle, and any high-altitude day trip — Rainbow Mountain above all — comes last of all.
For the wider picture, see our guides to Peru, Cusco and Lima.
When to go down
The rule that matters more than any other: if symptoms are getting worse, descend. Not tomorrow. Descent is the only reliable treatment, and in this region it is unusually easy — the Sacred Valley is an hour from Cusco and several hundred metres lower.
Do not climb higher with symptoms. Do not leave someone confused or breathless at rest to sleep it off. Everything else in this article is optimisation; this part is not.
This article is general travel information, not medical advice. Discuss altitude with a doctor before you travel, particularly if you have heart or lung conditions, are pregnant, or are travelling with young children.
Sources
- CDC Yellow Book: High Elevation Travel and Altitude Illness
- UNESCO World Heritage Centre: Historic Sanctuary of Machu Picchu
- Wilderness Medical Society: acute altitude illness practice guidelines
- UK FCDO: Peru travel advice
Last reviewed: September 13, 2026
