Taking two or more different drugs together may result in an alteration in the effectiveness or in the side-effects of one or more of the drugs. Some drugs should not be taken in combination with certain antiretrovirals.

It is important that your doctor and pharmacist know about all other medicines and drugs that you are taking – this includes those prescribed by another doctor, over-the-counter drugs, herbal and alternative treatments, and recreational drugs.

Some drug combinations are contraindicated – which means you definitely should not take them together. Reasons for this include serious side-effects, or interactions which make one or both drugs ineffective or toxic. 

Other interactions are less dangerous, but still need to be taken seriously. Levels of one or both drugs in your blood may be affected and you may need to change the doses you take. 

Your HIV doctor and pharmacist will check for possible interactions before they prescribe a new drug for you.

If any other healthcare professional prescribes or recommends a medicine for you, it’s important that they know about the drugs you are taking for your HIV. For example, it’s known that treatments for erectile dysfunction (such as Viagra) can interact with types of anti-HIV drugs that belong to the drug classes protease inhibitors (PIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs). Interactions with protease inhibitors can increase blood levels of Viagra and similar drugs, increasing the risk of side-effects.

You also need to tell your HIV doctor about any drugs you buy over the counter (at a chemist, for example) or from the internet. Some anti-HIV drugs can interact with antihistamines, asthma drugs (steroids), treatments for indigestion and statins (drugs that are used to control cholesterol, or lipid levels). These treatments can either be prescribed or bought over the counter at high-street chemists.

If you are thinking of using any other drugs, you should tell your HIV doctor or pharmacist so they can check for possible interactions and recommend the most suitable treatment. Or, when you are buying them, you may wish to tell the pharmacist about the anti-HIV drugs you are taking. High-street chemists often have a private area for consultations, or you could write the name of the drugs down and hand them to him or her. If you do need to mention the name of your anti-HIV drugs, it’s very unlikely that anyone around you will recognise what they are used to treat.

Less is known about interactions with recreational drugs. But there are potential interactions between some recreational drugs (for example, ketamine, ecstacy and methamphetamine [crystal meth]) and some NNRTIs and PIs. If you use recreational drugs, it is sensible to discuss this with your doctor, HIV pharmacist or other healthcare provider.

Anti-HIV drugs can also interact with herbal and alternative treatments. In many cases, the interactions are theoretical, or seen in test-tube studies, and more information is needed about the likelihood of a real-life effect.

St John’s wort, a herbal remedy used to treat anxiety and depression, lowers blood levels of NNRTIs and PIs.

Test-tube studies have indicated that African potato and Sutherlandia may reduce levels of PIs, NNRTIs and maraviroc (Celsentri) in the body.

Interactions can also happen with medicines that are not taken by mouth. For example, ritonavir can interact with inhalers and nasal sprays containing fluticasone and salmeterol (e.g. Flixotide, Flixonase, Seretide and Serevent), causing serious side-effects. 

You can take painkillers, such as paracetamol or ibuprofen (e.g. Nurofen), when on anti-HIV drugs, unless there are other medical reasons why you shouldn’t take this sort of drug.

Make sure you tell your clinic doctor and HIV pharmacist about all the medicines you are taking. This includes prescribed medicines, medicines you buy from a chemist, herbal or traditional medicines, and recreational drugs. Also check before taking anything new (whether you buy it yourself or have it prescribed by a doctor or dentist).