Side effects: when it's normal and when it's a warning sign

Reading the side effects list of any medicine is an experience that can frighten anyone. Nausea, dizziness, headaches, liver changes, serious allergic reactions — it sounds like the medicine does more harm than good. But there is a fundamental context that leaflets rarely explain clearly: the actual frequency with which each effect occurs, and above all, what to do when it happens.

How to interpret frequency terms in the leaflet

Regulatory authorities require that adverse effects be classified by frequency of occurrence. These terms have precise meanings that completely change the perspective on actual risk:

• Very common: affects more than 1 in 10 patients, i.e. more than 10% of users. If you take this medicine long enough, you are likely to experience this effect.
• Common: affects between 1 and 10 in 100 patients, between 1% and 10%. Possible, but not the majority of users.
• Uncommon: affects between 1 and 10 in 1,000 patients, between 0.1% and 1%. Less likely.
• Rare: affects between 1 and 10 in 10,000 patients, between 0.01% and 0.1%. Very unlikely for you specifically.
• Very rare: affects fewer than 1 in 10,000 patients, less than 0.01%. Statistically, you are more likely to be struck by lightning.

Effects that are expected and usually pass on their own

Many side effects are transient — they appear at the beginning of treatment and disappear as the body adapts to the medicine. Knowing them prevents you from abandoning an important treatment over something that would resolve naturally within a few days:

• Nausea in the first doses: very common with antibiotics, Metformin and antidepressants. Usually passes in three to seven days. Taking the medicine with a light meal tends to help considerably.
• Initial dizziness with antihypertensives: the body needs a few days to adapt to the new blood pressure level. Getting up slowly from bed and avoiding sudden changes in position minimises episodes.
• Dry mouth with antihistamines and antidepressants: a direct effect of the mechanism of action of these medicines, it may persist throughout treatment but is rarely dangerous.
• Drowsiness with anxiolytics and some antihistamines: especially intense in the first weeks. Avoid driving or operating machinery until you have assessed your level of alertness with the medicine.
• Diarrhoea with antibiotics: almost universal, as the medicine disrupts the intestinal flora. Probiotics taken a few hours after the antibiotic can help minimise the effect.

Signs requiring immediate contact with a doctor or pharmacist

• Any skin rash appearing after starting the medicine, even if it seems mild: hives can be the first sign of an allergic reaction that may progress.
• Dark urine the colour of strong tea, very pale or whitish stools, and yellowing of the skin or eyes: signs of possible liver involvement.
• Irregular, very fast heartbeats or a feeling of the heart pounding out of the chest after starting the medicine.
• Mental confusion, disorientation or unusual behaviour, especially in elderly people: may indicate toxicity or drug interaction.

Emergency signs — go to the emergency room immediately

• Sudden swelling of the lips, tongue, throat or face accompanied by difficulty breathing or swallowing: angioedema, a serious allergic emergency.
• Severe difficulty breathing, wheezing or a feeling of suffocation after taking the medicine.
• Skin with red patches forming blisters, especially around the mouth and genitals: may indicate Stevens-Johnson syndrome, rare but potentially fatal.

A simple habit that can save lives

When starting any new medicine, photograph the tablet or capsule with your phone and note the date you started treatment. If any unusual symptom appears in the following days, you will have documented exactly when treatment began — valuable information for the doctor to assess whether there is a causal relationship between the medicine and the symptom.