The question is rarely whether to mention a side effect. It is how fast. This page sorts medication side effects into four levels of urgency: emergency now, same day, a pharmacist this week, and a routine review. If you are unsure between two levels, act on the more urgent one.
For the general picture, see the medication side effects hub.
Common side effects and what they may feel like
Common examples linked to when to see a doctor for side effects
Most effects that bring people to search are in the mild band: nausea, headache, drowsiness, dizziness on standing, dry mouth, constipation or loose stools, and disturbed sleep. These usually do not need a doctor in the first instance. A pharmacist can tell you whether they are expected for your medicine and whether anything simple helps.
They do warrant an appointment when they persist beyond the adjustment period described in the leaflet, when they stop you taking the medicine as prescribed, or when they affect your ability to work, drive or sleep.
How to describe frequency without overclaiming
Bring observations rather than conclusions. What you took, when you took it, when the symptom started, whether it follows each dose, and whether it is improving. That sequence is what a clinician uses to judge whether the medicine is likely to be responsible.
Serious side effects and red flags
Red-flag symptoms
Emergency care now. Do not phone for advice first, and do not wait to see whether it settles:
- Difficulty breathing, wheeze, or a tightening throat
- Swelling of the face, lips, tongue or throat
- Chest pain, fainting or collapse
- Seizure, or sudden confusion or unresponsiveness
- A rash that blisters or peels, or sores in the mouth or eyes with fever
- Vomiting blood, or black tarry stools
- Thoughts of harming yourself
Same day. Contact a doctor today rather than booking ahead:
- Yellowing of the eyes or skin, dark urine, or pale stools
- Passing much less urine than usual, or marked swelling of the legs
- Fever with a new rash, or fever with mouth ulcers
- Unexplained bruising, or bleeding that is new for you
- A fast, slow or irregular heartbeat that is new
- Severe or worsening abdominal pain
- New muscle pain with weakness, or unusually dark urine
When to contact a doctor, pharmacist, or emergency service
A pharmacist is the right first contact for an over-the-counter product, a suspected interaction, or a question about whether an effect is expected. No appointment is needed and they can see the full product information.
Your prescriber is the right contact for anything involving a dose change, a medicine you have taken for a long time, a reaction that may mean you should not take that medicine again, or a decision about stopping.
Risk factors, interactions and who should be cautious
People who may need extra caution
For some groups the threshold for seeking advice should be lower, because effects are more likely and harder to spot early:
- Older adults. Slower clearance and more medicines at once make effects more likely and harder to attribute. See side effects in older adults.
- Kidney or liver conditions. Both organs clear medicines. Reduced function means an ordinary dose can behave like a larger one.
- Pregnancy and breastfeeding. Some medicines that are routine otherwise are not appropriate, and the reverse is also true. Never stop a prescribed medicine on this basis without advice.
- Children. Dosing is by weight and some medicines are not suitable at all. See children and over-the-counter medicines.
- People taking several medicines. Risk rises with the number of products, and includes anything bought without a prescription.
- Anyone with a previous drug reaction. A past reaction to one medicine can indicate risk with related ones.
Medicine and supplement interactions to check
The combinations most often missed are the ones people do not think of as medicines: supplements, herbal products, and anything bought over the counter. Tell a pharmacist about all of them, not only what was prescribed.
- Supplements and prescription medicines, including St John’s Wort, which affects how many medicines are broken down
- Supplements with blood thinners, where bleeding risk adds up
- Over-the-counter medicines alongside chronic medication
- Two products that both contain paracetamol, a common accidental overdose
- Alcohol with chronic medication
What to do if you think this is a side effect
Common effects
Keep taking the medicine unless you have been told otherwise, note the timing, and raise it with a pharmacist. Do not add another over-the-counter product to manage the effect before checking, because that is how interactions start.
Serious effects
Get help at the level the symptom calls for, and take the packaging with you. Do not abruptly stop a prescribed medicine on your own, because withdrawal from some medicines carries its own risk. See should I stop taking my medicine.
When to speak to a doctor or pharmacist
Red-flag symptoms
The emergency list above applies regardless of which medicine is involved or how long you have taken it. A medicine you have tolerated for years can still cause a new reaction.
When to contact a doctor, pharmacist, or emergency service
Have the box with you when you make contact. Strength and formulation matter, and brand names differ between countries.
Related side-effect guides
Common effects
- Medication side effects hub
- Serious medication side effects
- What to do if a medicine causes side effects
- Side effect symptoms index
- Report a side effect in South Africa
Serious effects
See urgent medication side effects and when side effects are serious if you are still weighing up urgency.
Medical Disclaimer: This content is for informational and educational purposes only and does not constitute medical diagnosis, prescription, or professional advice. Always read the patient information leaflet and consult a qualified doctor or pharmacist regarding any medicine or health concern.