Anaesthesia explained

Published: 01/03/2021

Preparing for an operation

As soon as you know that you may be having an operation, it is helpful to think about how you can improve your recovery from surgery by improving your health and lifestyle and increasing your activity.

The time that you have to prepare will vary depending on how urgent your operation is. This is particularly important for major surgery. The best outcomes will come from you working with your medical team in the weeks and months before the surgery. If your operation is an emergency, your doctors and nurses will give you good care in the condition you are in.

Our Fitter Better Sooner resources will provide you with the information you need to become fitter and better prepared for your operation. 

Medical problems 

If you have a long-term condition, such as diabetes, asthma or bronchitis, heart problems, are overweight or have high blood pressure, you should make sure you are as well as possible before your operation. If you think you could make some improvements, you can ask your GP for an extra check-up. You should do this as soon as you know that you are having an operation.


If you smoke, you should consider giving up before your operation. Smoking reduces the amount of oxygen in your blood and increases the risk of breathing and heart problems during and after the operation. Your surgical wound may heal more slowly and may be more likely to get infected if you smoke. On average, smokers stay in hospital longer than non-smokers.

The time before an operation is ideal to quit smoking. You may need the support of a specialist stop-smoking service. Your GP will be able to give you contact details for services in your area. You are four times more likely to give up successfully if you have this kind of help. For more information please go to:

Your weight 

Many of the risks of having an operation are increased if you are very overweight. Reducing your weight will reduce many of the extra risks you face during your anaesthetic and after your surgery. It may also make the surgery easier. Your GP can give you advice about weight loss and put you in touch with an organisation that can help. Slow, supervised weight loss is likely to be most successful.

To read more about how your weight can affect anaesthesia, please see our Anaesthesia and your weight leaflet.

Alcohol and other substances

If you drink more than the recommended amount, you should cut down before an operation. Go to the NHS website for more advice:

For your own safety, your anaesthetist needs to know if you are on the contraceptive pill or have used any recreational drugs.

Your teeth 

Ideally, loose and broken teeth should be assessed and treated before an anaesthetic as this reduces the risk of any damage to your teeth or them becoming displaced. On the day of your operation, your anaesthetist will want to know which teeth are loose, have crowns or a bridge.

Cardiopulmonary exercise testing (CPET)

Before major surgery, if you have medical risks your surgeon may ask you to do a CPET test before you decide that you will have the operation. This gives important information about your level of fitness, your risk and how well you will tolerate and recover from a major operation. This helps you decide whether an operation is right for you or whether other lower-risk treatments might be better for you. The test also helps your surgeon and anaesthetist decide how best to look after you during and after the operation.

The CPET test is usually done on a stationary cycle. You will be asked to wear a mask so the team can measure the oxygen that you use and the carbon dioxide that you breathe out.

If you cannot turn the pedals due to problems with your legs, it may be possible to use a machine that you turn with your arms.

Sometimes the exercise test team will suggest an activity programme designed to improve fitness to help you get through your operation safely.

This would only be if the operation is not an emergency and can be safely delayed.

Makeup, nail varnish and false nails

Makeup or body lotions are best avoided as they prevent heart monitor pads and dressings from sticking properly. Please remove nail varnish and ask for advice about false nails.

Some types of false nail interfere with monitoring the oxygen level in your blood. This is because a clip is used which usually shines a light through your fingernail.


You should remove jewellery and/or any decorative piercings ideally before you go to hospital. If you cannot remove it, the nurses will cover it with tape to prevent damage to it or to your skin. A wedding ring can usually be worn. It is important to remove any piercings around the lips and tongue before an anaesthetic to prevent difficulties and injury from the equipment used to look after your airway and breathing.

The preoperative assessment clinic

Before your anaesthetic we need to know about your general health. Some hospitals use a health questionnaire or you may be invited to a preoperative assessment clinic. Alternatively, you may receive a telephone or virtual assessment such as a Skype or Zoom video call. If you are having an urgent operation, the health check will be done by your doctors and nurses on the ward.

In most preoperative assessment clinics you will see a specialist preoperative assessment nurse. An anaesthetist may also see you, especially if you are having a major operation.

Tests that you need will be arranged for you. These may include blood tests, an ECG (electrocardiogram or heart tracing), an x-ray, a swab for the bacterial infection MRSA or other tests. Some tests can be done in the clinic, but for others you will need to come back on another day.

This is a good time to ask questions and talk about any worries that you may have. If the staff in the clinic cannot answer your questions, they will help you find someone who can.

Health-check information  

You may be asked about:

  •  your activity and any physical and mental health problems you might have
  • any serious illnesses you have had
  • problems with previous anaesthetics
  • problems you know about when people in your family have had an anaesthetic
  • symptoms relating to the heart or lungs such as chest pain, shortness of breath, palpitations or dizzy spells
  • heartburn or indigestion pain
  • medicines that you take, including those you can buy over the counter and vitamins or herbal remedies
  • allergies and reactions (please bring any details you have of the name of the drug and the reaction, including any letters if you have any)
  • smoking habits and the amount of alcohol you drink, and
  • use of recreational drugs such as cocaine or marijuana, heroin or the so called ‘legal highs’.

Pills, medicines, inhalers, and herbal remedies

An accurate list of your medicines is very important. Please bring with you either the pills themselves or a copy of your prescription from your GP.

Delaying your operation 

If your operation is not urgent, the anaesthetist or nurse at the preoperative assessment clinic may talk to you about taking some time to improve your health. More tests may be needed or some treatment may need to be started. They would do this working closely with your surgeon.

It is also possible that the anaesthetist you see may think that there are very high risks if you have the operation. You may want time to think about whether to go ahead with the operation.

Taking part in a clinical trial

Anaesthetists are always trying to improve the care that you receive. Clinical trials are studies of groups of patients that allow treatments to be compared. You may be asked to take part in a trial. A trial nurse or doctor will explain in detail what is involved and your consent will be sought. If you agree, you will receive full written information. It is your decision whether to take part, and you can withdraw at any time. If you can help, other patients in the future will benefit from the results of the trial.

Benefits and risks of having an anaesthetic

Anaesthesia has made modern surgery possible. Complex operations can be performed with a high degree of comfort and safety. However, there are risks associated with having an anaesthetic. These may be weighed up against the likely benefits of the operation.

Everyone varies in the risks they are willing to take. Your anaesthetist will describe the risks to you, but only you can decide how much the risk affects your plan to have the operation you would like.

Thinking about risk

The risk to you as an individual depends on:

  • whether you have any other illnesses
  • personal factors, such as whether you smoke or are overweight, and
  • whether the operation is complicated, long, or is an emergency procedure. To understand the risk fully you need to know:
  • how likely it is to happen
  • how serious it could be, and
  • how it can be treated if it happens. The anaesthetist can also advise you whether there are any anaesthetic techniques that will reduce these risks.

Side effects and complications

Anaesthetic risks can be described as side effects or complications. These words are somewhat interchangeable, but are generally used in different circumstances, as shown below.

Side effects are the unwanted but predictable and expected effects of drugs or treatments. For example, sickness is a side effect of a general anaesthetic, although steps are taken to prevent it.

Complications are unwanted and unexpected events due to a treatment. However, they are recognised as events that can happen. An example is a severe allergic reaction to a drug, or damage to your teeth when inserting a breathing tube. Anaesthetists are trained to prevent complications and to treat them if they happen.

See the section of our website for more information on anaesthesia and risk.

Common events and risks that healthy adult patients of normal weight face when having a general anaesthetic for routine surgery are summarised in our risk infographic.

For specific information on the different risks involved in anaesthesia, please see the following leaflets which are all available here.

In the few days before your operation


You should continue to take your medicines up to and including the day of the operation unless you are told not to.

Please follow carefully the instructions you have been given in the preoperative assessment clinic. Please look out for specific instructions if you take:

  • drugs to thin your blood (for example, warfarin, dabigatran, rivaroxaban, clopidogrel, aspirin)
  • drugs for diabetes
  • all herbal remedies, and
  • some blood-pressure pills.

Taking a shower

Some hospitals give patients a disinfecting shower gel to use for several days before the operation. It helps to prevent serious infections such as MRSA. You should use this gel on your body and hair as directed.

Packing a bag for hospital

Whether you are staying overnight or whether your operation is day case, it is a good idea to bring comfortable clothes and footwear. Temperatures in hospital can vary so some layers of clothing can be useful. See NHS Choices for further information on what to pack for hospital: