There are a variety of ways in which people can experience adverse reactions to medications, whether prescribed or 'over-the-counter'. Most of these effects are not 'allergy'.
Some people are more affected by drugs than others; there is a large degree of individual variability. Some people excrete drugs more slowly than others, and levels of the drug may build up over time. This means that a dose of a drug that suits one person might be too much (or too little) for another.
There are unlikely to be toxic effects, as the doses prescribed take account of this variation and allow a wide margin of safety. However, it may be that a sedative that makes someone else sleep for a few hours might make another person sleep for much longer. This would be an increased effect rather than an allergy or intolerance.
All medications have side-effects because of the way they work. The majority of people get none, or very few, but some people are more prone to them. The most common side-effects are rashes, itching, nausea, vomiting, diarrhoea (or occasionally constipation), lethargy, headaches and blurred vision. All the known side-effects from a drug are listed in the patient information. They are listed in order of frequency (most common will be first on the list).
If you suffer a rash and itching after taking a drug, it can be hard to decide if this is a side-effect or an allergy. Morphine commonly causes itching, but this is not an allergy and the drug can still be used. In children, many rashes experienced while taking antibiotics are thought to be due to the underlying viral illness.
Many drugs interact in some way with others. Some interactions are so severe that the different drugs are never given together, but many are just 'warnings' - there may be some odd symptoms but not in most people. If you are taking more than one drug and experience odd symptoms, check whether an interaction could be to blame (ask your pharmacist or GP).
Some drugs are not supposed to be used in certain illnesses. This may be because they will not be eliminated properly, for example, in people with kidney or liver disease; because they may cause problems for the foetus in a pregnant woman; or because they are known to cause side-effects in particular illnesses (for example, amoxicillin often causes a rash when given to someone with glandular fever, but not when used in other conditions).
Some people are genuinely allergic to a drug, but this is quite uncommon. Most drug 'reactions' are due to the various mechanisms described above.
Some drugs, such as penicillin, some anaesthetics, vaccines and other injections used in, for example, X-ray techniques, are known to be more likely to cause allergic reactions. They are used more cautiously in allergic people. If you suffer a fairly severe adverse reaction to a drug, or a repeated reaction on different occasions, this will normally be considered to be an allergy. This should be recorded in your medical notes and you should not be given the drug again. In each group of drugs, there are some that are less likely to cause allergy problems.
If you suffer a reaction and are not sure what drugs you were given, ask your GP or hospital Doctor for the information (for hospital Doctors, contact their secretary). If you have any trouble getting the information, ask your local NHS PALS organisation to help (see the phone book or hospital / surgery notice boards).
Blood tests are not usually very helpful in determining drug allergies. Most allergies are very specific - if you react to one antibiotic this does not mean you are more likely to react to other antibiotics, although you should not be given one of the same type. If you are concerned, ask for a small test dose of a new drug before you are given the normal amount. Some drugs such as anaesthetics and vaccines can be checked by skin testing, although this usually involves injecting a very small amount of the drug under the surface of the skin.
Drug testing is complicated so always seek the advice of an Allergy Specialist or other Specialist Doctor who is experienced in diagnosing and managing medicine allergies.
If you are allergic to a drug, take responsibility for making sure everyone knows. Health services are still not very good at passing this sort of information on, or looking at it even when it is there. Ask whether the allergy has been recorded in your notes. Volunteer the information to everyone who is involved in your care, even if they don't ask. Make sure you are given an allergy bracelet in hospital. If your allergy is severe, consider wearing your own allergy bracelet. If you are given drugs to take at home, read the label and contents carefully.