Most people only look this up once the sickness has come back sooner than they expected. The last tablet went down 5 hours ago, the box says every 12 hours or every 8, and the obvious question is whether one more would hurt. In the UK, Zofran is usually dispensed as plain ondansetron, and for most adults the gap between doses will be 8 or 12 hours, depending on what it was prescribed for. UK product information puts the adult ceiling at 32 mg in 24 hours, and people with severe liver disease shouldn’t go above 8 mg a day. Whether an extra tablet is risky will depend less on the number on the box than on your heart, your blood salts and what else you are taking, so it’s worth going through those before reaching for the packet again.
What The Leaflet Will Have Told You
The spacing changes with the reason you were given it, and that’s why two people on ondansetron can have quite different instructions.
- For chemotherapy or radiotherapy, the usual adult pattern is 8 mg taken 1 to 2 hours before treatment, then 8 mg every 12 hours, for no more than 5 days.
- For harsher chemotherapy, a hospital may give a single dose of up to 24 mg together with a steroid called dexamethasone before the session starts.
- Around an operation, it might be one 16 mg dose an hour before the anaesthetic, or 8 mg followed by two more 8 mg doses at 8 hourly intervals.
- Children on chemotherapy are dosed by size, sometimes up to 3 doses 4 hours apart on the first day, then 2 mg or 4 mg every 12 hours after that.
Plenty of ondansetron is also prescribed for things that aren’t on that list at all, such as a bad stomach bug, migraine, or sickness in pregnancy. That’s normal practice, but it means the schedule on your label came from your GP or the hospital doctor rather than from the product information, so their instruction is the one to follow. If the label simply says “every 8 hours when required”, take it as meaning every 8 hours at the very closest.
When The Last Dose Has Worn Off Early
So you’re back at hour 5, feeling grim, with tablets in the cupboard. Should you take one?
We’d say no, not without asking someone first, and the reason isn’t only caution. Ondansetron works by blocking one particular set of receptors, and once those are covered, extra drug doesn’t do a great deal more. In surgery studies, 8 mg by drip worked no better than 4 mg. A second tablet at hour 5 will probably leave you just as sick, only with more of the side effects.
What’s more likely to help is a different kind of anti-sickness medicine. A pharmacist or your GP might suggest cyclizine, or prochlorperazine as a buccal tablet (Buccastem), which you tuck high up between your gum and upper lip and leave to dissolve, so it still works if you can’t keep anything down. Metoclopramide is another option, although in the UK it’s normally limited to 5 days because of the risk of movement side effects. If you were given ordinary tablets and keep bringing them back up, ask about the orodispersible version too, since it dissolves on the tongue without water.
Threw up straight after taking a dose? Don’t automatically take another. You’ll have no way of knowing how much was absorbed, so ring the pharmacy, or NHS 111 out of hours, and ask.
Why Your Heart Is The Real Limit
Ondansetron can lengthen a part of the heartbeat known as the QT interval. Most of the time nothing comes of it, but a long QT can occasionally tip into a dangerous rhythm that makes people faint, and in rare cases it can be fatal.
Regulators had to take this seriously in 2012, when a study showed a single 32 mg dose into a vein lengthened the QT by around 20 milliseconds on average, and that one big intravenous dose was withdrawn. Since then, single doses by drip have been kept far lower. Tablets taken at home weren’t changed, because they’re lower doses and absorbed more slowly.
The people most tempted by an extra dose are often the people whose hearts will cope with it least. Two days of vomiting or diarrhoea will have washed out potassium and magnesium, and low levels of both make QT problems more likely. Others who should be especially wary:
- anyone born with long QT syndrome, who generally shouldn’t have ondansetron at all.
- people with heart failure or a slow heart rate.
- anyone already on medicines that also lengthen the QT, which include citalopram, some antibiotics such as clarithromycin, methadone and certain heart rhythm drugs.
If you feel your heart pounding or racing, come over dizzy, or black out after taking it, ring 999 or go to A&E. Don’t wait to see if it settles.
To be fair to the drug, it has a wide margin in healthy people, and hospital mix ups have seen patients given doses as large as 150 mg by drip without serious harm. Those patients were on a ward with monitors and nurses around them, though. Someone who’s been sick for 2 days at home, with their salts already low, isn’t in the same position at all.
Liver Disease And The 8 mg Limit
Ondansetron is broken down almost entirely by the liver. In a healthy adult a dose will have mostly cleared within the day, but in severe liver disease its half life stretches out to roughly 20 hours, so a second tablet lands on top of the first instead of replacing it. That’s why the limit drops to 8 mg for the whole day. Milder liver problems don’t usually need any change.
A Few Other Things That Change The Answer
Taking ondansetron alongside antidepressants such as sertraline or venlafaxine, or with migraine triptans, has occasionally been linked to serotonin syndrome, where people get agitated, sweaty, shaky and hot with a fast pulse. More doses raise that chance a little. Constipation and headache are the side effects most people notice, and they’ll usually get worse the more you take.
If you’re pregnant, the MHRA has said that taking it in the first 3 months carries a small extra risk of a cleft lip or palate, about 3 extra cases in every 10,000 pregnancies. Your midwife or GP can talk through whether it’s still the right choice for you, and you shouldn’t stop or double up on your own either way.
Jot down the time of every dose, on your phone or on the box itself, and if 2 doses in a row wear off early, ring your GP or the pharmacy that same day rather than closing the gap yourself.
