After-drop is a further fall in core body temperature after you leave cold water, as cold blood from the arms and legs returns to the centre of the body. Dry quickly, put on warm dry layers with a windproof outer, take a warm non-alcoholic drink, and stay with someone. Treat shivering that will not stop, confusion or slurred speech as an emergency.
What after-drop is
A 2024 paper in BMJ Open Sport & Exercise Medicine describes after-drop as a continued fall in deep body temperature during rewarming, attributed to cold blood returning to the core from the periphery. The authors call it a serious complication of cold water immersion that in extreme cases can lead to ventricular fibrillation, a dangerous heart rhythm. They also say research on rewarming is limited, while cold shock and hypothermia are well documented.
The University of Portsmouth editorial on minimising risks in cold water therapies adds a practical point: cooling continues for around 30 minutes after leaving the water, so avoid driving during that period. That is why the hour after a dip matters as much as the dip.
How big is the effect
In the 2024 study, 15 healthy men sat head-out in still water at about 14°C for 30 minutes. Five minutes into rewarming, deep body temperature had dropped by about 0.16°C to 0.18°C across the three rewarming conditions (a changing robe, a towel and a foil blanket). The drop in that setting was small.
Do not read too much into that. The study was small, men only and in a controlled lab, using still water and passive immersion. Real swims involve wind, exertion, wetsuits or none, and a long wait on a beach. The study was funded by a changing-robe company, though the authors state it had no role in the research. Treat it as one data point, not a guarantee.
A cautious rewarming routine
The following draws on the BMJ paper's recommendations (a hot drink, a warmer place, warm dry clothing), the Portsmouth editorial (dry and dress quickly, windproof outer layer) and NHS first aid for hypothermia. It is general information, not medical advice.
- Get out before you are too cold. Numb hands make dressing slow and unsafe.
- Dry yourself quickly, hair included, and take off wet swimwear.
- Put on dry layers in a sensible order, with a hat and a windproof outer. A long changing robe works well over the top while you dress. See changing robes and post-swim kit.
- Move out of the wind to shelter or a warm car. Keep the car for warming up, not driving, for the first stretch.
- Take a warm, non-alcoholic drink and something sugary to eat if you are fully alert.
- Stay with a companion until you have stopped shivering and feel normal. Mild shivering is expected; shivering that is getting worse is not.
What to avoid
- Alcohol. The NHS advises against giving alcohol to someone who is cold.
- Very hot baths or showers when you are already shaking or unwell. NHS guidance says a hot bath, hot water bottle or heat lamp will not help someone with hypothermia and could make things worse. For a mild chill after a dip the sources consulted give no official advice on hot showers, so a warm rather than hot shower is a cautious reading, not a rule.
- Rubbing arms or legs. The NHS lists this as something not to do for hypothermia.
- Driving soon after. Portsmouth's editorial advises against it in the first 30 minutes or so.
- Going straight back for another round while you are still cooling.
When it is an emergency
The NHS lists the symptoms of hypothermia in adults as shivering, pale cold dry skin with lips or skin turning blue or grey, slurred speech, slow breathing, and tiredness or confusion. It calls hypothermia a medical emergency that needs hospital treatment and says to call 999 or go to A&E. In Ireland the emergency numbers are 999 and 112.
While waiting for help, the NHS advice is to move the person to shelter, remove wet clothing and wrap them in a blanket or dry towel with their head covered, give a warm non-alcoholic drink and sugary food if they are fully awake, keep them talking, and stay with them. If someone has collapsed or is not breathing normally, call 112 or 999 straight away and follow the call-handler's instructions. For more on helping someone else, see what to do if someone is in trouble in the water.
The more you cool, the longer rewarming takes. If you notice you are slow to recover after your usual dip, shorten the next one. Read ice bath temperature and duration and the safety checklist.
Pack for the exit, not just the entry
Plan the end of the session before the start. Lay out clothes in the order you will put them on, keep a warm drink in a flask, and choose clothes that are easy to manage with cold fingers: loose layers, no tight cuffs, no small buttons. Neoprene gloves, boots and a cap can help keep extremities warmer in the water; see neoprene gloves, boots and caps. If you have a heart or circulation condition, speak to your doctor first, because rewarming can also stress the circulation.
Frequently asked questions
What is after-drop?
It is a continued fall in core temperature after you leave cold water, as cooled blood returns from the limbs. It can happen even when you feel better.
How long should I wait before driving?
A University of Portsmouth editorial says cooling continues for around 30 minutes after leaving the water and advises against driving in that period.
Should I have a hot shower straight away?
The NHS says hot baths do not help hypothermia and could worsen it. For a mild chill the sources give no firm rule, so choose warm, not hot.
When is shivering a warning sign?
When it will not stop, or comes with confusion, slurred speech or blue-grey skin. The NHS says to call 999 for suspected hypothermia.