Dehydration (脱水症, *dasshui-shō*) is one of the most preventable yet commonly encountered health problems among international tourists in Japan, particularly during the June–September summer period. Japan's summer is characterized by:
High temperatures: Tokyo regularly exceeds 35°C (95°F) in July and August
Extreme humidity: 70–90% relative humidity, making sweating less effective at cooling the body
Urban heat island effect: Cities like Tokyo, Osaka, and Kyoto trap heat between buildings and concrete surfaces
Extensive walking: Japanese sightseeing involves significantly more walking than many tourists anticipate — 15,000–25,000 steps per day is common
Japan's heat has become increasingly dangerous. The country experiences thousands of heatstroke-related hospitalizations and hundreds of deaths each year, with the summer of 2023 seeing record-breaking heat and emergency room crowding.
For foreign visitors, additional risk factors include:
Not being acclimatized to Japanese summer heat (arriving from cooler climates)
Not understanding how intensely dehydrating Japan's humidity is
Being unfamiliar with Japanese rehydration products and where to find them
Prioritizing sightseeing over rest in air-conditioned environments
Underestimating fluid loss from sweating in outdoor queues at attractions
The good news: Japan is exceptionally well-equipped to address dehydration. OS-1 oral rehydration solution is available at every convenience store and pharmacy, vending machines with cold drinks are ubiquitous (approximately one per 23 people nationally), and public air-conditioned spaces are widely available as cooling shelters.
Dark urine, fatigue, headache, dizziness, dry mouth
Rest, ORS, monitor
Severe
5–8% body weight
Very dry mouth, no urine 8+ hours, extreme fatigue, rapid heart rate, confusion
Seek medical care immediately
Children: Children become dehydrated more rapidly than adults and require proportionally larger fluid volumes. Watch for wet diapers as a hydration indicator in infants.
Elderly travelers: Reduced thirst sensation makes elderly individuals especially vulnerable to underhydration even without feeling thirsty.
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Heatstroke (熱中症, *netchū-shō*) in Japan encompasses a spectrum from heat exhaustion to life-threatening heatstroke:
Level 1 — Heat Cramps / Fainting:
Muscle cramps in legs/abdomen
Brief loss of consciousness (standing or exercising in heat)
Cold, pale, sweaty skin
Treatment: Move to cool area; ORS; rest; monitor
Level 2 — Heat Exhaustion:
Severe fatigue, weakness
Headache, nausea, vomiting
Dizziness; may vomit
Skin: heavy sweating, pale
Temperature: normal to mildly elevated
Treatment: Cool environment, ORS, rest; visit clinic if not improving rapidly
Level 3 — Heatstroke (Medical Emergency):
Confusion, disorientation, or loss of consciousness
Hot, red skin (may have stopped sweating — a dangerous sign)
High core temperature (40°C+)
Rapid, strong pulse
Possibly seizures
Treatment: Call 119 immediately; cool the person with cold water and fanning while waiting for ambulance
OS-1: Japan's Oral Rehydration Solution
OS-1 (オーエスワン) is a medically formulated oral rehydration solution (ORS) manufactured by Otsuka Pharmaceutical. It is the gold standard for treating dehydration in Japan and is available at:
Every convenience store: 7-Eleven, FamilyMart, Lawson (500ml bottles, approximately ¥150–200)
All pharmacies and drugstores: Matsumoto Kiyoshi, Welcia, Sundrug
Supermarkets
Hospital pharmacies
Why OS-1 is better than water or sports drinks for dehydration:
Drink
Sodium (mEq/L)
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OS-1's low glucose and higher sodium content optimizes the sodium-glucose cotransporter mechanism in the intestine, dramatically improving water absorption compared to high-sugar sports drinks.
Pocari Sweat (ポカリスエット) and Aquarius are widely available alternatives that are better than water but not as effective as OS-1 for true dehydration. They are appropriate for mild dehydration prevention during physical activity.
Prevention: Staying Hydrated During Japan Sightseeing
Practical hydration strategies:
Drink before you feel thirsty — thirst is already a sign of mild dehydration
Target 250–500ml of fluid per hour during outdoor activity in summer
Use Japan's vending machines (自動販売機, *jidōhanbaiki*) strategically — they are everywhere, and cold drinks are available for ¥100–150
Carry a refillable water bottle — water fountains exist in parks and at public facilities; Japan's tap water is safe to drink
Take air-conditioned breaks every 45–60 minutes of outdoor activity. Convenience stores (コンビニ), department stores, and shopping malls are excellent free cooling shelters
Wear light, breathable clothing and a hat; carry a portable fan (扇子, *sensu*, or uchiwa)
Avoid peak heat hours (11am–4pm) for the most strenuous sightseeing; schedule indoor activities midday
Carry ORS (OS-1) packets or bottles when visiting theme parks, temples, outdoor markets
Alcohol and dehydration:
Japan's beer garden culture, sake bars, and widespread availability of alcohol contribute to dehydration. Alcohol is a diuretic — alternate alcoholic drinks with water or ORS, and ensure you hydrate well before sleep after an evening out.
Which Department to Visit and How to Find a Clinic
If you need medical care for moderate to severe dehydration:
内科 (Naika) — Internal Medicine: appropriate for dehydration with or without associated illness
救急外来 (Kyūkyū Gairai) — Emergency: for severe dehydration, suspected heatstroke, or altered consciousness
Call 119 for ambulance if someone has collapsed, is confused, or cannot be roused
IV Drip (点滴, *tenteki*) at outpatient clinics:
Many Japanese neighborhood internal medicine clinics offer IV fluid administration as an outpatient service. If you are significantly dehydrated but not unconscious, you can walk into a clinic and request IV fluid therapy. A 500ml saline drip typically takes 30–60 minutes and provides rapid recovery. This is common practice in Japan and is much faster than queuing at a hospital emergency room.
Use Traveler's Hospital to find internal medicine clinics near your location that offer IV drip therapy.
Cost Estimate
Service
Approximate Cost (No Insurance)
Internal medicine clinic consultation
¥3,000–¥6,000
IV fluid therapy (500ml drip, clinic)
¥5,000–¥10,000
Blood electrolyte test
¥2,000–¥5,000
Emergency room treatment
¥15,000–¥50,000+
Ambulance service
Free in Japan
OS-1 (500ml, convenience store)
¥150–¥200
Pocari Sweat (500ml, vending machine)
¥130–¥160
Note: Ambulance transport in Japan is free of charge — you will not be billed for the ambulance ride itself.
Children and Dehydration in Japan
Children are at higher risk of rapid dehydration and heatstroke. Japanese summer is especially intense for children, who often want to continue playing regardless of heat.
Monitor children's urine color: pale yellow is good, dark or absent is a warning sign
Use OS-1 child-appropriate amounts (consult packaging or pharmacist)
Apply sunscreen and hats, and take frequent shade breaks
A: OS-1 is available at virtually every convenience store (7-Eleven, FamilyMart, Lawson) in Japan, all pharmacies, and most supermarkets. It is sold in 500ml bottles and 1.5L bottles. If you cannot find OS-1 specifically, ask store staff or look for "経口補水液" (oral rehydration liquid) signs.
Q: Is Japan's tap water safe to drink?
A: Yes, completely. Japan's municipal tap water meets strict safety standards and is safe to drink throughout the country. Refilling a water bottle from the tap is a cost-effective way to stay hydrated while sightseeing.
Q: How do I recognize heatstroke in myself or a companion?
A: Key warning signs of serious heatstroke: skin that is hot and red but has stopped sweating (a danger sign), confusion or difficulty speaking coherently, inability to walk without support, extremely high body temperature, and loss of consciousness. These signs require an immediate call to 119 (ambulance) and active cooling (cold water, fanning).
Q: Is the Japan summer really that dangerous for tourists?
A: Japan's summer heat is genuinely dangerous and should not be underestimated. Japan regularly records thousands of emergency heatstroke hospitalizations each summer. Tourists are at elevated risk because they are unacclimatized, walking more than usual, and unfamiliar with hydration resources. Proper preparation — early starts, midday rest, constant hydration, ORS availability — makes summer tourism in Japan perfectly manageable and enjoyable.
Q: Is Pocari Sweat as good as OS-1 for dehydration?
A: Pocari Sweat and Aquarius are better than plain water for dehydration, but OS-1 has a more optimal electrolyte-to-sugar ratio for treating actual dehydration. For mild prevention and activity hydration, sports drinks are fine. For moderate dehydration or dehydration from illness (vomiting/diarrhea), OS-1 is the medically recommended choice.
Q: When should I call an ambulance (119) instead of going to a clinic?
A: Call 119 if the person: cannot be woken up or is unresponsive, is severely confused or not making sense, is having a seizure, has collapsed and cannot stand, or has a very high fever (above 40°C) with confusion. For moderate dehydration where the person is alert and able to walk, an internal medicine clinic or urgent care clinic is appropriate.