Indinavir originally entailed a three times a day ‘fasted’ dosing regimen, but now it is mainly used with ritonavir.

Ritonavir boosts the levels of indinavir so that it can be taken twice daily, with or without food.

Indinavir is mainly processed through the kidneys (most drugs are cleared through the liver) and one of the side effects is a build up of indinavir crystals in the kidneys. About 20% of people will have evidence of indinavir crystals, with 4–10% of people using indinavir showing symptoms of kidney blockage.

This is why you need to drink at least 1.5 litres of water a day (about three pints or six large glasses), especially just after taking your medications. This helps the tiny crystals of indinavir flush cleanly through your kidneys.

The risk of a blockage is related to the peak levels of indinavir. If the drug levels are too high, or if you don’t drink sufficient water, then a blockage can be caused because the crystals can accumulate as a sludge.

This is not the same as a real kidney stone, but the symptoms are still very similar: stomach cramps, bladder pain and, most predictably, a dull pain or ache that can quickly develop into an extremely sharp pain in your lower back. Dark urine, or urine containing blood can indicate kidney stones.

A kidney blockage is very painful and very serious and requires immediate attention. If untreated a blocked kidney can lead to irreversible damage.

If you use 400mg of indinavir with 400mg of ritonavir for each of your twice-daily doses, you will have lower peak levels of indinavir and can use 1.5 litres of water.

If you use higher indinavir doses (usually 800mg or 600mg) with smaller doses of ritonavir (100mg or 200mg) then you will have a higher peak level of indinavir and greater hydration may be important.

In hot weather, and after exercise, increase your water intake even more. Tea, coffee and alcohol will cause you to dehydrate, so do not include these when adding up your fluid intake.

If you have these symptoms, try to drink as much water as you can (and sit upright or stand up to try to help any blockage to clear).

Acidic drinks like orange juice and cranberry juice can help as indinavir is more soluble in acidic conditions. If the pain gets worse seek immediate medical advice.

With an indinavir-related blockage, treatment is through increasing fluid intake (by intravenous drip and drinking) together with painkillers to control the pain.

If the problem takes more than 1–2 days to clear – or is very painful – you may want to stop all your drugs until it clears up. A single treatment interruption of a few days is unlikely to cause problems.

If you don’t want to stop treatment, consider switching indinavir for another PI or an NNRTI for the next few days to help the blockage clear.

Once you have cleared the problem it is safe to continue to take indinavir again – especially if it was working well for you – but make sure you keep drinking sufficient fluid.