Space motion sickness and motion sickness

A 39-year-old astronaut makes her first orbital flight. A few hours after reaching microgravity, she feels unwell, turns pale and breaks into a cold sweat, loses her appetite and vomits twice. She notices that the symptoms worsen when she turns her head quickly to look at a panel or when she sees a colleague floating upside down relative to her. She trained on parabolic flights and had never been so nauseated. The mission doctor, in contact with the crew, must decide whether this is space motion sickness and whether it is worth treating now. The scene is fictional and was written for teaching.

This text uses the scenario as a setting for clinical reasoning: what space motion sickness is, why the conflict between the senses explains much of the picture, what the sources say about the differential, what doses of promethazine were used in flight and in research, and why adaptation usually comes within a few days. The text does not replace medical assessment or the protocols of each space agency. If you play the case of the day on NEXO, you will recognize the challenge: a common symptom, vomiting, in a context that changes the weight of each hypothesis.

Key points

• Space motion sickness is a form of motion sickness and not a separate diagnosis.
• It affects most astronauts on their first flight and usually improves within a few days.
• The conflict between the signals from the inner ear, vision and body explains a lot, but not everything.
• Intramuscular promethazine is the treatment described in NASA flight series.
• Systematic reviews do not allow pointing to one countermeasure as superior to the others.

Why nausea in space is a reasoning scenario

Lackner and DiZio describe space motion sickness as a form of motion sickness, not as a separate diagnosis. Proposed causes include fluid shift to the head, head movements, visual illusions, Coriolis effects and asymmetry between the otoliths on the two sides (Lackner and DiZio, 2006). A systematic review by Khalid and colleagues estimates that the syndrome affects about 70 in every 100 astronauts, with nausea, dizziness, fatigue, vertigo, headache, vomiting and cold sweat (Khalid and colleagues, 2023).

The scenario is useful because it combines a rare environment with a common complaint. Nausea and vomiting appear in dozens of conditions, but on the first day in orbit one of them is much more likely than the others. The exercise is to use the context without letting it erase the other questions.

What the scene teaches

The scene has five clues. The first is timing: symptoms in the first hours of microgravity. The second is the trigger: rapid head movements and disorienting visual scenes. The third is autonomic signs, such as pallor and cold sweat. The fourth is the first flight. The fifth is previous experience on parabolic flights without the same nausea. The post on signs, symptoms and history shows how the trigger of a symptom guides the diagnosis.

How many astronauts get sick

Davis and colleagues reviewed 24 space shuttle flights. On the first flight, 57 of 85 crew members had space motion sickness. Of all cases, 26 were mild, 20 moderate and 11 severe. Among crew members who flew a second time, the frequency fell to 46 in every 100, a difference that was not statistically significant (Davis and colleagues, 1988). The authors also saw differences by sex, on-board role and age, but none were significant.

In the Skylab missions, Homick reported that symptom recovery was complete in 3 to 5 days. After the eighth day of the mission, all crew members were immune to experimentally induced nausea (Homick, 1979). This data is important for reasoning: a condition that improves within a few days is consistent with adaptation, and a condition that worsens or persists calls for other questions.

More than nausea and vomiting

Lackner recalls that motion sickness is a complex syndrome, with many components beyond nausea and vomiting, and that, under normal conditions, many cases go unrecognized (Lackner, 2014). Golding recounts that the word nausea itself comes from the Greek naus, ship, and describes nausea and vomiting as the main signs of a condition that can be triggered on land, at sea, in the air and in space (Golding, 2016). In the scenario, this means also asking about drowsiness, apathy, headache and difficulty concentrating, which can weigh on work even before the first vomit. The symptom list in the review by Khalid and colleagues includes fatigue, vertigo, headache and cold sweat (Khalid and colleagues, 2023).

Pathophysiology: the conflict between the senses

Homick recalls that research at the time into the cause of space motion sickness relied mainly on the sensory conflict theory. The idea is that the brain receives signals from the inner ear, vision and body that do not match what it expects. In microgravity, the otoliths, which normally report the direction of gravity, stop providing that reference (Homick, 1979).

Lackner and colleagues showed that altered control of head and body posture, together with altered vestibular and motor control, also causes motion sickness. This helps explain both nausea in space and the disturbances that appear on return after long flights (Lackner and colleagues, 1991).

What the theory does not explain

Lackner, in a later review, states that there is still no fully adequate theory of motion sickness. Sensory conflict explains many aspects, but not all. Studies in parabolic flight pointed to the velocity storage of signals from the semicircular canals as an important factor both in space and on Earth (Lackner, 2014). The same review notes that only people without a functioning vestibular system are fully immune.

The conflict between canals and otoliths

Weerts and colleagues describe space motion sickness as caused by a conflict between the semicircular canals and the otoliths. Their hypothesis is that depressing canal function with drugs could relieve the symptoms, and so they tested the effect of various drugs on each part of the labyrinth (Weerts and colleagues, 2014). This line of research shows how pathophysiology guides the choice of treatment, even when the answer is not yet ready.

Who is more susceptible

Susceptibility varies widely among healthy people. Golding describes that it also changes considerably across patient groups, including people with different types of vestibular disease and people with migraine (Golding, 2016). Lackner adds that sleep deprivation increases susceptibility (Lackner, 2014).

In the astronaut's case, parabolic flight training did not guarantee protection. The sources read do not include a ground test able to predict who will get sick in orbit, and Homick already noted in 1979 that there were no satisfactory methods of prediction, prevention and treatment (Homick, 1979).

Sleep and the first night in orbit

The first night in orbit is usually short and restless, and sleep deprivation increases susceptibility (Lackner, 2014). The sources read do not measure the sleep of the astronaut or of crews in general on the first day, and so this post does not include numbers on that.

The differential diagnosis in orbit

The sources read for this text do not include a list of other causes of vomiting in orbit, and so this post does not present one. What they allow is to separate some questions the mission physician needs to ask.

The fluid hypothesis

The shift of fluids toward the head was one of the first explanations proposed. Homick reports that research up to then did not support this hypothesis as a cause of space motion sickness (Homick, 1979). Lackner and DiZio still cite it among the proposed causes (Lackner and DiZio, 2006). For the reasoning, this means that the puffy face and nasal congestion of the first day, common in orbit, neither confirm nor rule out the diagnosis.

Radiation

NASA's technical summary on radiation protection lists acute radiation syndromes among the risks during the mission, with nausea, vomiting and fatigue, tied to solar particle events. The document states that effective shielding and environmental monitoring reduce this risk (NASA OCHMO-TB-020). In the scenario, the useful question is whether there was a solar event alert. With no alert and with clear motion triggers, motion sickness remains the most likely hypothesis.

Medication effects

The anti-nausea drugs themselves are part of the picture. The Cochrane review of antihistamines found sedation in 66 out of every 100 people on an antihistamine, versus 44 out of every 100 on placebo (Karrim and colleagues, 2022). An astronaut who gets more drowsy after treatment may be having a drug effect, not a worsening of the illness. The post on diagnostic uncertainty discusses how to keep more than one hypothesis open.

Assessment on board

The sources read do not define tests for space motion sickness. Davis and colleagues graded cases as mild, moderate and severe using a standardized questionnaire given by the flight surgeon after the mission (Davis and colleagues, 1988). Khalid and colleagues, in the systematic review, criticize precisely the lack of a standardized form of assessment and the small studies (Khalid and colleagues, 2023).

In the scenario's practice, the assessment relies on the history: when the symptoms started, what makes them worse, how many times there was vomiting, whether the astronaut can drink and work, and whether there was any unexpected event. The post on pretest probability helps to think about how the context of the first day changes the probability of each hypothesis.

Questions that organize the history

With no tests defined by the sources, the history needs to be well organized. A useful sequence starts with timing: at what hour after arrival in microgravity the first symptom appeared. Then comes the trigger: whether the discomfort gets worse with head movements, with visual scenes in which "up" and "down" change, or with tasks that require looking up close. Next, severity: how many times there was vomiting, whether the person can drink, eat and carry out tasks. Finally, what else happened: medications taken, the previous night's sleep and any environmental alert. This sequence follows the causes proposed by Lackner and DiZio and the susceptibility factors described by Lackner (Lackner and DiZio, 2006; Lackner, 2014).

Management: promethazine and when to treat

NASA's experience changed over time. Davis and colleagues compared treatment strategies on space shuttle flights. Oral prophylaxis with scopolamine and dextroamphetamine delayed or did not prevent symptoms. Intramuscular promethazine relieved symptoms within up to 12 hours in 14 of 15 crew members, and the suppository in 6 of 8. Without treatment, recovery was slow, over 72 to 96 hours. Because of these results, and partly because oral absorption is variable early in the flight, NASA began treating after symptoms start (Davis and colleagues, 1992). The summary of this study does not report the dose used.

Bagian and Ward reviewed 21 crew members treated with intramuscular promethazine. Only 1 reported sedation, while ground studies described sedation in 60 to 73 out of every 100 people. The authors note that NASA has used intramuscular promethazine for space motion sickness since 1989 (Bagian and Ward, 1994).

What parabolic flight and ground studies say

Abbott and colleagues tested oral promethazine in parabolic flight. In the placebo group, there was nausea in 51.8 out of every 100 parabolas; in the promethazine group, in 12.6 out of every 100 (Abbott and colleagues, 2025). Four of six participants on promethazine had no nausea at all (Abbott and colleagues, 2025). It is a small study in a short-duration environment, not a clinical regimen for orbit.

Weerts and colleagues studied on the ground the effect of meclizine, of dimenhydrinate with cinnarizine and of promethazine with d-amphetamine on the semicircular canals and the otoliths (Weerts and colleagues, 2014). The authors describe promethazine as a vestibular suppressant and recall that, despite being the treatment used, it has inconsistent efficacy in space motion sickness. These are experimental studies, not recommendations for use; that is why this post does not give doses.

Antihistamines and scopolamine on the ground

For motion sickness in general, the Cochrane review by Karrim and colleagues concluded that first-generation antihistamines probably prevent symptoms under natural conditions. 40 out of 100 people had no symptoms with an antihistamine, against 25 out of 100 with placebo, with a relative risk of 1.81 and moderate certainty (Karrim and colleagues, 2022). The review found no studies on treating symptoms that had already started and almost no data in children (Karrim and colleagues, 2022).

The Cochrane review by Spinks and Wasiak concluded that scopolamine is better than placebo for prevention, with no trials on treating symptoms that had already started. Dry mouth was more common with scopolamine (Spinks and Wasiak, 2011). The abstracts of these two reviews do not give doses, and that is why this post does not cite them.

Adaptation and the limits of the evidence

Adaptation is the rule. Homick reported complete recovery in 3 to 5 days and immunity to induced nausea after the eighth day of the mission (Homick, 1979). Lackner describes that adaptation procedures developed to increase resistance to motion sickness reduce the time constant of velocity storage (Lackner, 2014).

The course of the case

In the scene, the mission physician advises the astronaut to reduce rapid head movements and to keep a stable visual reference while she works. She receives intramuscular promethazine, per the mission protocol, and by the end of the second day she is already eating and carrying out her tasks. On the third day, the symptoms disappear. The course matches the adaptation described in flight series and supports the initial hypothesis. If she had gotten worse, or if fever, localized abdominal pain or a solar event alert had appeared, the physician would have had to reopen the diagnosis. The continuation is also fictional.

The return to Earth

Adaptation to space has a cost on the way back. Lackner and colleagues link altered sensorimotor control not only to nausea in space but also to the disturbances that appear on return after long flights (Lackner and colleagues, 1991). For clinical reasoning, the lesson is that the same mechanism can produce symptoms at both ends of the mission.

The second flight

Davis and colleagues observed that 9 of the 26 crew members who flew twice had milder symptoms on the second flight, but there was no difference in the mean interval between flights for those who had and those who did not have space motion sickness. The authors suggest that differences in training and flight experience may explain part of the variation (Davis and colleagues, 1988). Having felt nauseated once, therefore, does not define what will happen on the next mission.

The evidence has clear limits. The review by Khalid and colleagues concluded that pharmacological and non-pharmacological measures may help, but that no definitive recommendation can be made about the superiority of one approach (Khalid and colleagues, 2023). Lackner recalls that there are still no adequate remedies against motion sickness without adverse effects (Lackner, 2014).

Reasoning: the context that helps and the context that blinds

The context of the first day in orbit is a strong clue. The probability of space motion sickness is high, and the astronaut's history matches it. The risk is failing to ask what the context does not explain, such as the absence of improvement within a few days, a solar event alert or new symptoms after a remedy.

Croskerry describes premature closure as the tendency to accept a diagnosis before confirming it (Croskerry, 2003). The post on examples of heuristics in clinical reasoning shows how useful shortcuts can become traps, and the post on the diagnostic pause teaches you to stop before closing the case.

Another point in the scenario is the difference between the experience in training and the real experience. The astronaut did not get sick on the parabolic flight, and that could lead her to doubt her own diagnosis or to hide the symptoms. Lackner notes that vulnerability varies enormously between people and that many cases go unrecognized (Lackner, 2014). Asking directly and without judgment is part of the assessment, because a symptom reported late is a symptom treated late.

Case summary in a table

Case clue	What it suggests	What the source read says
Symptoms in the first hours	Space motion sickness	Most crew members on the first flight (Davis 1988)
Worse when turning the head	Sensory conflict	Head movements among the proposed causes (Lackner and DiZio)
Disorienting visual scene	Visual illusion	Visual illusions among the proposed causes (Lackner and DiZio)
First flight	Higher risk	Lower frequency on the second flight, without significance (Davis 1988)
Question about radiation	Rare acute syndrome	Linked to solar events, reduced by shielding (NASA)
Decision to treat	Treat after the symptoms	Intramuscular promethazine relieved 14 of 15 (Davis 1992)
Expectation of improvement	Adaptation within days	Recovery in 3 to 5 days (Homick)

How NEXO helps train this reasoning

To use the case of the day as training, try three habits. Before each guess, ask where the person was and what they were doing when the symptom began. When the context seems to solve the case, list what still needs explaining. After the result, note whether the context helped or got in the way.

Three exercises with cases

Weigh the context. Write down how much the environment changes the probability of each hypothesis.

Look for the trigger. Identify what makes the symptom worse and what makes it better.

Set the timeframe. Say how long the picture should take to improve if the first hypothesis is right.

The post on learning based on clinical cases shows how cases help fix this habit.

The limits on using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and under creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Is space motion sickness a form of motion sickness?

Yes. Researchers describe it as a form of motion sickness, not as a separate diagnosis.

Do most astronauts have symptoms on the first flight?

Yes. In the space shuttle flight series, most first-time crew members had symptoms.

Do the symptoms usually improve within a few days?

Yes. On the Skylab missions, recovery was complete in 3 to 5 days.

Does sensory conflict explain much of the picture?

Yes. It explains many aspects of motion sickness, though not all of them.

Is intramuscular promethazine used to treat the condition in flight?

Yes. The NASA series describe the use of intramuscular promethazine after the symptoms begin.

Does sleep deprivation increase susceptibility?

Yes. Lackner's review cites sleep deprivation as a factor that increases susceptibility.

Final checklist

• I asked when the symptoms began in relation to arrival in microgravity.
• I identified the head movement and visual scene triggers.
• I recorded the frequency of vomiting and the ability to drink and work.
• I asked whether it was the first flight and how the training went.
• I remembered sleep deprivation as a susceptibility factor.
• I asked about a solar particle event alert.
• I considered the sedation caused by the medications themselves.
• I reviewed the promethazine dose described in the flight series.
• I set the expected timeframe for improvement through adaptation.
• I followed the space agency protocols for treatment and follow-up.

Want to practice reasoning with progressive clues? Play the case of the day.

References

1. Abbott R, Weinrich MM, Keller NW, et al. Promethazine Effects on Motion Sickness During Altered Gravity Induced by Parabolic Flight. Aerosp Med Hum Perform. 2025. PMID 41253367.
2. Bagian JP, Ward DF. A retrospective study of promethazine and its failure to produce the expected incidence of sedation during space flight. J Clin Pharmacol. 1994. PMID 9766972.
3. Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003. PMID 12915363.
4. Davis JR, Jennings RT, Beck BG. Comparison of treatment strategies for Space Motion Sickness. Microgravity Q. 1992. PMID 11541442.
5. Davis JR, Vanderploeg JM, Santy PA, et al. Space motion sickness during 24 flights of the space shuttle. Aviat Space Environ Med. 1988. PMID 3240221.
6. Golding JF. Motion sickness. Handb Clin Neurol. 2016. PMID 27638085.
7. Homick JL. Space motion sickness. Acta Astronaut. 1979. PMID 11902170.
8. Karrim N, Byrne R, Magula N, et al. Antihistamines for motion sickness. Cochrane Database Syst Rev. 2022. PMID 36250781.
9. Khalid A, Prusty PP, Arshad I, et al. Pharmacological and non-pharmacological countermeasures to Space Motion Sickness: a systematic review. Front Neural Circuits. 2023. PMID 37396400.
10. Lackner JR. Motion sickness: more than nausea and vomiting. Exp Brain Res. 2014. PMID 24961738.
11. Lackner JR, Dizio P. Space motion sickness. Exp Brain Res. 2006. PMID 17021896.
12. Lackner JR, Graybiel A, DiZio PA. Altered sensorimotor control of the body as an etiological factor in space motion sickness. Aviat Space Environ Med. 1991. PMID 1930058.
13. NASA Office of the Chief Health and Medical Officer. NASA-STD-3001 Technical Brief: Design for Ionizing Radiation Protection, OCHMO-TB-020 Rev G. 2025. Document.
14. Spinks A, Wasiak J. Scopolamine (hyoscine) for preventing and treating motion sickness. Cochrane Database Syst Rev. 2011. PMID 21678338.
15. Weerts AP, Vanspauwen R, Fransen E, et al. Space motion sickness countermeasures: a pharmacological double-blind, placebo-controlled study. Aviat Space Environ Med. 2014. PMID 24919385.