The most serious allergic reactions tend to follow injections, especially those given straight into the veins (intravenous injections). This is because the drug is carried around the body very quickly in the blood.
If you are allergic to the drugs given as part of a general anaesthetic that have a paralysing effect, signs such as flushing, difficulty breathing or a drop in blood pressure can appear within seconds, usually within three minutes. Approximately 500 people have serious allergic reactions to anaesthetic drugs every year in the UK, that’s one in every 10,000 procedures. Your anaesthetist will be looking out for the signs of a reaction and will be ready to treat it.
Injections given beneath the skin (subcutaneous) or into a muscle (intramuscular) can cause a local reaction at the injection site, including reddening, swelling (weals) and itching.
Reactions to drugs that you take by mouth can start within minutes but might happen up to two hours later if the drug is absorbed slowly. Some delayed-release drugs may cause reactions that come on many hours later.
Through the skin
Antibiotics or other drugs applied to burns or inflamed or damaged skin may cause allergic reactions. On rare occasions they can cause anaphylaxis.