What are side effects and why do they occur?

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This page includes general questions about side effects.

What are side effects?

Medicines are developed to treat a medical problem. But even when they are very effective, they sometimes have other effects.

These are called side effects. Sometimes they are called adverse events or referred to as drug toxicity.

Every drug is likely to have some side effects for some people, even if this is very rare. In most cases side effects are mild and easy to manage.

Sometimes they are so mild that they are not noticed. Side effects to ART usually only affects a minority of people.

Serious side effects to current ART are rare.

Do all drugs have side effects?

Every drug is likely to have some side effects for some people, even if this is very rare. In most cases side effects are mild and easy to manage.

Sometimes they are so mild that they are not noticed. Side effects to ART usually only affects a small proportion of people.

Serious side effects to current ART, although possible, are not common.

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.

How common are side effects with ART?

Modern HIV drugs (ART) are very safe and very effective.

In general, the risk of serious side effects is very low. And if you do have problems it is easy in the UK to use a different treatment.

Sometimes there are mild side effects when starting any new medicine. But often these settle down within a few weeks as we get used to them.

Regular monitoring on treatment will also pick up any long-term side effects.

A few recent side effects are also not fully understood. For example, whether some more recent drugs are linked to increased in weight in some people.

Symptoms vs side effects

It is important to remember the difference between a side effect and a symptom.

A symptom is used for any change in how you feel that you could report to your doctor. For example, feeling tired, or having diarrhoea are both symptoms that could be side effects.

Often though, many symptoms might be caused by something else. Even though we think  things are side effects it is important to work with your doctor to check this.

Other side effects can only be seen after a lab test, for example, high cholesterol or raised liver enzymes. But these can also be caused for other reasons than a side effect.

The symptoms of many common side effects are similar to symptoms of illnesses. Your doctor needs to know about every symptom in order to be able to decide whether it is caused by treatment (a side effect) or a different illness.

Different treatments are needed when a symptom relates to an illness.

How are side effects reported?

Information about side effects from HIV drugs comes from research studies. These often report a daunting list of possible side effects – even though you are very unlikely to get any of them.

However, some side effects occur in some people. This is why you will be carefully monitored when you are on treatment. It is also why it is important to talk to your doctor about any problems you have or any changes you notice.

Information about any risk should include both how likely it is to occur and also how serious the symptoms might be.

The risk should be given in real (numerical) terms. This is so you have factual information when choosing HIV treatment.

For example, if this is 1 in 10 chance that it will occur (10% risk). 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. See Table 2.

Even though language is very important, 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.

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

Some side effects are only discovered after a drug has been approved.

Sometimes, side effects that are only recently discovered might not be included in the printed information about the drug.

However, most drugs become safer over time. As more people use them, more information is collected.

Where can I get more information?

Every medicine, including ART, should come with a leaflet. 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.

Last updated: 1 November 2021.