What Causes a Sickle Cell Crisis? Triggers to Watch For
A sickle cell (vaso-occlusive) crisis can be brought on by dehydration, extreme cold or heat, high altitude or low-oxygen environments, infection, physical overexertion, and stress — though according to both the NHLBI and NHS, a crisis can also occur with no identifiable trigger at all. Knowing the common ones doesn't guarantee prevention, but it does make it easier to lower your risk and recognise an episode early.
What is a sickle cell crisis?
A sickle cell crisis, most often a vaso-occlusive crisis, happens when misshapen red blood cells block blood flow through small vessels, causing sudden and often severe pain. It's the most common complication of sickle cell disease and the leading reason people with the condition seek emergency or urgent care (NHLBI). Crises vary widely in severity and location — pain commonly affects the chest, back, arms, legs, or abdomen, and can shift between locations even within a single episode.
What causes a sickle cell crisis? The common triggers
Health authorities point to a consistent set of factors that raise the likelihood of a crisis, even though any individual episode may not have a clear cause:
- Dehydration — reduces blood volume and can promote the sickling process (NHS). See how much water to drink with sickle cell
- Extreme temperatures — both very cold and very hot conditions, including sudden temperature changes (see our guides to cold weather and hot weather)
- Low oxygen environments — high altitude, unpressurised flights, or strenuous exercise at intensity beyond what you're used to
- Infection and illness — the body's response to infection can trigger a crisis, which is part of why vaccinations and prompt treatment of infections are emphasised for people with sickle cell disease
- Stress — both physical and emotional stress are recognised contributing factors
- Overexertion — pushing physical activity well beyond your normal level, especially without adequate hydration and rest
These factors don't act alone — several often combine, such as intense exercise in hot weather without enough fluids. That's also why a crisis can happen without any single obvious cause: the buildup isn't always visible in the moment.
What are early warning signs to watch for?
Pain is the most recognizable warning sign, but it's often not the only one. Other things worth paying attention to, according to the NHLBI, include unusual fatigue, fever, swelling (particularly in the hands or feet, known as dactylitis, which is often an early sign in young children), shortness of breath, and pain that feels different in character or location from your usual episodes. Noticing these early — rather than waiting until pain is severe — can make a real difference in how quickly you get treatment started.
Can you prevent every crisis?
No, and it's worth being clear about that rather than implying triggers are fully controllable. Some crises happen with no identifiable cause at all, and even careful management of known triggers won't eliminate risk entirely (NHLBI). What consistent attention to hydration, temperature, infection prevention, and overexertion does is lower the odds and, often, the frequency — not guarantee against every episode. Long-term treatments prescribed by a haematologist, such as hydroxyurea or other disease-modifying therapies, also play a significant role in reducing crisis frequency for many people and are worth discussing directly with your care team.
What should you do if you think a crisis is starting?
Start basic self-care right away: rehydrate, rest, and use any pain management approach your care team has already recommended for early-stage pain. Keep track of how the pain is progressing — location, intensity, and anything unusual alongside it, like fever or swelling. If pain is severe, doesn't respond to your usual approach, or is accompanied by other worrying symptoms such as high fever, difficulty breathing, or confusion, seek emergency care rather than waiting it out. Having an emergency plan and key contacts ready before a crisis starts — not scrambled together during one — makes a real difference in how quickly you can act.
- you think you're having a sickle cell crisis and have severe pain, breathing difficulties or are being sick
- you have sickle cell disease and a high temperature
- you have an erection that is painful or does not go away (priapism)
Do not drive yourself to A&E. (NHS)
Fill in your plan and download it as a PDF.
Frequently asked questions
This guide is general information, not medical advice. Always follow the plan agreed with your own care team. Read our medical disclaimer.
Related guides
Yes, cold and sudden temperature changes can trigger a sickle cell crisis. Why it happens, why to use heat instead of ice packs, and a winter checklist.
A simple way to log sickle cell pain — what to record, how to rate it 0–10 — plus a free printable pain diary template to take to your next appointment.
How heat and dehydration can trigger a sickle cell crisis, how to stay cool and hydrated in hot weather or a warm climate, and when to get help.