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Missed a dose?

Friday, 15 May 2015 Written by // Guest Authors - Revolving Door Categories // Research, Health, Treatment, Living with HIV, Revolving Door, Guest Authors reports new research that says “For some HIV treatments, 85% adherence may be enough”

Missed a dose?

This article first appeared in HIV Update, a publication of, here.  

The best results of HIV treatment are seen in people who take their pills as prescribed – at the right times, without missing doses and following any instructions about food. But what happens if adherence is not quite perfect? What is the impact of sometimes missing a few doses in a month?

Previously research suggested that it was necessary for people to take at least 95% of doses at the right time and in the right way to have the best chances of achieving an undetectable viral load. But those studies were done several years ago. Some modern anti-HIV drugs may not require such high levels of adherence.

In particular, efavirenz and other drugs in the non-nucleoside reverse transcriptase inhibitor (NNRTI) class have long half-lives. This means that it takes quite a long time for the body to break down the drug. As a result, even when one dose is missed, there may still be enough of the drug in the bloodstream – left over from previous doses – to prevent the replication of HIV.

This is not generally the case for other anti-HIV drugs. For example, protease inhibitors such as atazanavir and darunavir have much shorter half-lives. They are broken down quickly by the body and missed doses will have more of an impact.

(If you are unsure which types of anti-HIV drug you are taking, NAM’s Antiretroviral drug chart may help you work it out.)

The new study analyses ten years of data from around 22,000 people living with HIV in the United States. Their adherence was estimated by looking at how often they went back to the pharmacy for antiretrovirals.

For people taking a protease inhibitor, adherence above 95% was associated with the greatest chance of having an undetectable viral load. Poorer outcomes were observed even when people were taking between 90 and 94% of their doses.

But for people taking NNRTI-based therapy, an adherence level of 85% or above was associated with just as high a chance of achieving an undetectable viral load as an adherence rate of 95% or above.

If you’re taking a once-a-day treatment, then 85% adherence would mean missing around four doses a month, while 95% adherence would involve missing one or two doses a month.

The study also found that more people taking NNRTIs were able to have good adherence than people taking protease inhibitors.

Good adherence remains the single most important thing that you can do to ensure that your HIV treatment works. Almost 100% adherence is the best thing to aim for. But the study shows that if you’re taking efavirenz, rilpivirine, Atripla or another NNRTI-based regimen, then a little forgetfulness may not be the end of the world.

NAM’s booklet ‘Taking your HIV treatment’ includes information on why adherence is important, what to do if you miss a dose and practical tips to help you remember.