top of page

Medicines and Drugs - Interactions and Side Effects


If anything on this web page makes you worry about the medicines you are taking, speak to your doctor and get their advice before you make any changes.


We have split these tips into At Home, At the Doctors and At the Pharmacy

Manage Medicines Safely - At Home

  • If you receive a prescription with unusual, unexpected, or confusing directions, ask your doctor.

  • Order your repeat medicines online using the NHS App

  • If you sometimes forget what to take when, you can either ask your pharmacist to use a dosette box, or use a medicines management App on your SmartPhone or Tablet

  • Don’t hoard medicines, they go out of date, are easier to muddle up, and you could end up taking them after your GP has de-prescribed them

  • Keep hydrated, consider a 'motivational water bottle'

  • #KnowYourNumbers consider regularly tracking your weight, pulse rate, blood pressure, blood oxygen and keeping a wellness diary so that you can monitor the effect of any medicines you are taking, and the success of any lifestyle changes you are making

  • Consider Asking for GP Record Access so you can check your doctor’s notes after a consultation to remind yourself about their guidance.

  • Consider asking for Proxy Access by your friend to your GP Record to help them help you organise your healthcare.

  • Consider using Patient Knows Best via the NHS App to let a friend help you organise your healthcare​

NHS App - Prescriptions Menu.jpg
Use a Dosette Box to manage medicines.png
Keep Hydrated to avoid Medecine Side Effects.png

Manage Medicines Safely - At the Doctors

  • Ask your doctor how you are supposed to take your medication before you leave the room, and make a note to remind yourself later

  • Ensure you know what each of your medicines is for - ask your doctor to write this down for you on your prescribing list, or add it to the prescription so this information is on the instructions for each medicine.

  • Attend for a medicine review with your doctor at least once a year.

  • You and your doctor together will work through all the medicines on your repeat prescription list, determining for each:

    • Is it needed? (What are you taking it for, is it working, is it still necessary?)

    • Is it causing any side-effects?

    • Are there any risks from taking it long-term?

    • Are you on the right dose? Could this be reduced and still be effective?

    • Do you need any tests, such as blood tests, to monitor that the medicine is not harming you or that the dose is right for you?

    • Does it interact with anything else you are on?

    • Do you want to continue taking it?

  • Consider taking a friend or family to your appointments to help you to record what the doctor says

Manage Medicines Safely  - At the Pharmacy

Your pharmacist is a very useful source of information and advice about medicines. They may often be able to answer questions you have about your medicines and save you having to make an appointment with the doctor. You may also be offered a 'medicines use review' by your pharmacist. This is another way to review all your medicines and if you are using them correctly, and the pharmacist may pick up issues your doctor is not aware of. A medicines use review may involve:

  • Checking you know what you are taking your medicines for.

  • Checking you know how to take them correctly, and are doing so.

  • Checking you are remembering to take the medicines regularly.

  • Finding out if you are having any side effects from your medicines or any difficulty taking them as recommended.

  • Making suggestions to you and/or your GP on how to reduce any problems you may have with your medicines (e.g. a dosette box if you are having difficulty remembering when to take all the different tablets, or suggesting an alternative if a certain tablet is difficult to swallow or giving you side effects).

  • Checking you are ordering enough of your medicines to take them regularly, and not too many, so they are wasted.

  • Checking you are happy to continue taking all the medicines you are on.

A pharmacist may suggest some of the following changes to help you:

  • a home delivery service which may help if you have difficulty shopping

  • a health check service to help with your long-term health conditions and see if your medications are working well

  • an online repeat prescription service where you can pick up medications without having to get a paper prescription from your GP first

  • pre-packaging your medications in single use foil dispenser so that you know what you should be taking at different times of the day.


  • Side Effect - doctors use the term ‘side effect’ for any unwanted effect on your body from a single drug you are taking,

  • Interaction - one drug you are taking might be interfered with by a second drug [or food, vitamins, a supplement, a non-prescription medicine].

  • PolyPharmacy - the term ‘polypharmacy’ is for when you are taking many medicines with possible interactions between them

We explain more below.

Polypharmacy - when too many medicines interact

Medicines are keeping us alive for longer, but as we live longer we have more medical problems for which we need medicines. Polypharmacy is the use of multiple medications at the same time by one person. In other words, it means being on lots of different medicines. Usually, a person who is on quite a few different pills has more than one medical condition. There is no single definition of polypharmacy. It isn't defined as being over a specific number of medicines, although being on six, seven or more medicines is often taken to suggest you could run into difficulties.

  • Medicines interact with each other. So one medicine may be less effective, or cause more harm when taken with another medicine

  • Medicines affect different people differently, so it isn't always possible to predict that a medicine will cause a concern.

  • The more medicines you are on, the more likely it is that at least one of them will cause either a side effect, or some kind of long-term damage.

  • If you get to the point where you feel so unwell from the side effects of your medicines that life is not enjoyable, it may no longer be worth taking so many

Falls - dizziness can be a symptom of polypharmacy

Falls are more common in people who are on multiple medicines, and these may have serious outcomes. After a fractured hip, for example, one in ten people die within the first month, and three in ten within a year. And finally, it is easy to end up in a spiral of more and more medicines. Once you start taking a medicine to counteract the side effects of another medicine, you are on a slippery slope

Work with your doctor and your pharmacist to make sure that any medicines you are on are necessary and are not causing you more harm than good.

Age UK Guide - Making the most of your medicines

The Age UK web pages below are well worth reading. Age UK say “As we get older, many of us find that we need to take at least one medication due to a long-term health condition. Although it can become routine, some people may have problems remembering to take a dose, opening child-proof bottles, using an asthma inhaler, or applying eye drops.

Your pharmacist can help you better understand your medicines and solve any medication challenges”

Drug Interaction Dizziness.jpg
Drug Side Effect Dizzy.jpg
Age UK PolyPharmacy Report.jpg


If you are really struggling with how your medicines are making you feel, and you would like to check drug interactions for yourself, there are some websites in the USA that provide a free drug interactions report.

Safety first:

  • This is very important: do not change the medications you are taking without speaking to your GP first

  • Secondly, please note this website is maintained free by health professionals in the USA. They have designed it for use in America only, it’s not licensed for use in the UK.

  • Bear in mind that your GP may not have heard of it.

It’s very simple to use, just enter the medicines you are on typing in one name at a time, and then press ‘check’.

Visit this site to try it out:

Medicines Interaction Check.jpg



Every drug has side effects. How vulnerable you are to these side effects depends on many different factors, which can be grouped as:

  • patient-related,

  • drug-related,

  • life or lifestyle related

Find out if you are more at risk from drug-related reactions, and what you can do to manage some of these side effects.

Your Risk of Developing Side Effects

Your age is the most significant of these factors. The very young and the very old are ALWAYS more susceptible to unwanted reactions.

Children are not small adults. The way their bodies absorb process and eliminate drugs differs from adults, and this is especially true in babies. Younger children tend to absorb medicine more slowly from the stomach but have faster intramuscular (IM) absorption rates. In early life, they have a higher body water to fat ratio and a larger liver to bodyweight ratio. Liver enzymes are immature, as is their kidney function. In addition, the permeability of their blood-brain barrier (the layer of cells that restricts the passage of substances from the bloodstream to the brain) is higher.

Older adults typically take more medicines and studies have shown they are twice as likely to go to A&E because of an adverse drug event, and seven times more likely to be hospitalised. They are more likely to be on medicines with a narrow margin between being effective or toxic such as warfarin, insulin, digoxin, and anti-seizure medications. Their bodies tend to have more fat and less water which may increase the duration of effect of certain drugs. In addition, processing in the liver and excretion through the kidneys is typically reduced. Their brains are also more sensitive to the sedating effects of drugs, and pre-existing problems, such as dizziness, eye, and ear problems, may be increased, making a fall more likely.

Individual Factors That Might Increase Your Risk of Developing Side Effects

Other factors also play a significant role in the likelihood of side effects, including:

Genetics: Genetic factors account for 20-95% of patient variability. This field of pharmacology is rapidly evolving and testing for liver enzyme variations becoming more widespread. For example, codeine requires metabolism through CYP2D6 for conversion to one of its active metabolites, morphine.

  • 5-10% of people are poor metabolisers - which means that in these people, very little codeine is converted to morphine which results in insufficient pain relief.

  • However, 1-2% of people are ultra-rapid metabolisers and more codeine is converted into morphine than normal, resulting in a higher risk of toxic reactions including respiratory depression.

Kidney function:  If your kidneys are not functioning at full capacity, then side effects are more likely if you are taking drugs that are excreted through the kidneys. Some other drugs may lose their effectiveness when kidney function is reduced.

Gender: Females have a lower activity of certain hepatic enzymes, a higher body fat to water ratio, and a decreased clearance of drugs through the kidneys than men. Studies have shown the incidence of drug-induced liver toxicity, gastrointestinal side effects, allergic skin reactions, and long QT syndrome is higher in females.

Drug-Related Factors Might Increase Your Risk of Developing Side Effects

Drug-related influences include:

  • The dose of the drug: The higher the dosage the greater the risk of side effects

  • The formulation used: For example, inhaled steroids directly target the lungs and produce fewer side effects than oral steroids that have more of a body-wide effect

  • How the drug is absorbed, metabolized, distributed, and eliminated

  • Other medicines that are being taken at the same time.

Your Life or Lifestyle Might Increase Your Risk of Developing Side Effects

  • Drinking habits: Alcohol can intensify some side effects such as drowsiness, confusion, changes in blood pressure, abnormal behaviour, respiratory depression, and nausea and vomiting

  • Smoking: The hepatic enzymes CYP 1A2 and 2B6 are induced by cigarette smoking. These enzymes metabolize several clinically important drugs (such as clozapine, olanzapine, and methadone), which means an abrupt cessation of smoking can increase blood levels of these drugs. Nicotine replacement therapy does not influence enzyme activity

  • What you eat: Several medicines interact with foods. For example, atorvastatin with grapefruit, green leafy vegetables and warfarin, natural black licorice and lithium, salt and potassium supplements and ACE inhibitors, and tyramine-containing foods and MAOI antidepressants

  • How much water you drink: Dehydration can increase the risk of side effects of some medicines. Some medicines that shouldn't be taken by a person who is dehydrated include ACE inhibitors, NSAIDs, metformin, and diuretics

  • Level of understanding about a medicine: Side effects are more likely if instructions are not clear, or a person is confused as to why they are taking a medicine; both of which may result in a wrong dosage being taken.

bottom of page