Why do side effects occur?

Side effects occur because the body is a very complex.

It is difficult to make a drug that targets one part of the body but that doesn’t affect other parts.

Developing drugs is also complicated because no two people are exactly the same. So even drugs that have virtually no side effects, might be difficult for some people.

Current drugs might not be perfect, but they are better than they have ever been. And drugs in development now will hopefully be better still.

New drugs are developed to be better than earlier treatment.

Where can I get more information?

Every medicine that you are prescribed, including ART, should come with a leaflet about each of the drugs. If your hospital doesn’t provide this then ask for it.

This leaflet is important. Even when the information is simplified, it should include:

  • How and when to take the drug.
  • Whether you need to take it with food.
  • Common and/or serious side effects.
  • Interactions with other drugs.

Sometimes the leaflet is much more detailed, usually in small print and is similar to the Summary of Product Characteristics (SPC).

The SPC is a detailed document for every new drug. It is available free on the European Medicines Agency (EMA) website in most EU languages.

The information in the SPC includes more detail about:

  • All reported side effects and their frequency in studies.
  • The studies that led to approval, and
  • Food and drug interactions, and doses, including dose changes.

Information on each HIV drug on the i-Base website includes a direct link to the EMA web page for that drug.

How are side effects reported?

The risk of side effects should always be given in real (numerical) terms. This is so you have factual information when chosing HIV treatment.

A 10% risk means you have a 1 in 10 chance that it will occur. This is the same as saying if 10 people use the drug, one person is likely to get the side effect.

Sometimes the risk is described with more general words, like rare, or common.

Language is very important but it is not always used correctly.

A side effect that occurs in more than one in 10 people is ‘very common’. A rare side effect has to occur in less than one in 1,000 people, see Table 2.

When a drug is first studied, every side effect is recorded, even if it cannot be directly linked to the drug being studied.

This is one reason why the leaflet that comes with any drug usually has such a long list of potential side effects.

The risk of getting most of these listed side effects is usually very low – often less than 1 in 100 or 1 in 1000.

If side effects only become apparent after the drug has been approved, the drug leaflet might not have this information.

Some side effects are only discovered after a drug has been approved. However, most drugs become safer over time, as more people use them, and more information is collected.

Table 2. Definitions for side effect frequency

Very common affects 1 to 10 people in 10: ie a 10% chance or higher
Common affects 1 to 10 people in 100: ie a 1% to 10% chance
Uncommon affects 1 to 10 people in 1,000: ie a 0.1% to 10% chance
Rare affects 1 to 10 people in 10,000: ie a 0.01% to 10% chance
Very rare affects less than 1 in 10,000 people: ie less than a 0.001% chance
Not known frequency cannot be estimated from the available data

Last updated: 1 August 2016.