University of Sydney researchers analysed information from the NSW Poisons Information Centre database, which fields calls nationally, as well as PBS prescribing data. They identified 17,299 poisonings from atomoxetine, clonidine, dexamphetamine, guanfacine, lisdexamfetamine and methylphenidate over the 10-year study period.
The poisoning rate increased by 16.5% per year on average—which broadly tracks with growth in PBS dispensing. But rates are rising faster in some groups, including baby boys and adolescent girls, highlighting the need for patient education and careful monitoring, experts say. More than half (56%) of cases involved hospital attendance.
Almost four in 10 cases were intentional, including recreational use, deliberate self poisoning and other intentional misuse. About 80% of the intentional poisonings involved deliberate self-harm.
Unintentional poisonings included therapeutic errors, “accidental exploratory exposures” aka curious toddlers ingesting medicine that wasn’t intended for them, pregnancy exposures and adverse reactions.
Methylphenidate and clonidine each caused 35% of poisonings overall, but clonidine caused more per prescription dispensed—6.5 per 1000 versus 1.0 per 1000 on average.
Clonidine poisonings are outstripping diagnosis rates, says the paper’s lead author Amy Thomson, a PhD candidate in the university’s School of Pharmacy.
Moreover, 70% of clonidine poisonings needed hospital care.
Overdose symptoms include bradycardia, hypotension, drowsiness and pinpoint pupils, Ms Thomson says.
Clonidine has a very narrow therapeutic window, she explains. “So any tiny mistake that happens can be quite serious.”
Dr Alison Poulton, a paediatrician specialising in ADHD and senior lecturer at the University of Sydney, says clonidine causes sleepiness, and is often used to counteract the side effects of stimulants. However, Dr Poulton says she tries not to prescribe it “because of the side effects and it’s because it’s known to be much more dangerous in overdose.”
“When it comes to sleep, I’ve always used melatonin because it’s just so much safer,” she says. “But I do think that we’re in line for a surge in clonidine use because of difficulty accessing melatonin. I have had parents who are no longer able to purchase melatonin online requesting a prescription for clonidine,” she says.
Clonidine is much more affordable, she notes, and GPs can prescribe it without restrictions. “It’s cheap for people to get, whereas melatonin is safer, but more expensive on prescription, possibly less effective. But I worry much less about significant adverse consequences with melatonin.”
In September the TGA warned against purchasing melatonin online from the US where it’s sold over-the-counter due to concerns about overdose, and American online retailer iherb has suspended sales of the product in Australia.
Paradoxically, although not recommended in guidelines, clonidine may be prescribed off-label to children with ADHD aged 4-6 who are considered too young to be prescribed stimulants.
As NSW considers whether to restrict GPs from prescribing stimulants for ADHD to kids under 6, Dr Poulton worries about unintended consequences.
“Assuming that doctors do not treat children without a good cause, restrictions on stimulants may lead them to prescribe the less safe and less effective alternative – clonidine,” Dr Poulton argues.
Dr Bryce Joynson—a GP with extensive experience in ADHD management—says part of the problem with clonidine is that it’s a tiny tablet that has to be cut to give children an appropriate dose.
“The standard starting dose is usually 25 or 50 micrograms, and one tablet is 100 micrograms. So we might give them a quarter or half a tablet. It’s quite hard to break because it’s so small. So it’s quite easy to give children too much.”
“They might get a full one, and then the parent might forget and give them another one. They’ve now had 200 micrograms. You can see how quickly we can overdo the dose.”
Seventy-two percent of poisonings occurred in people up to the age of 19 years, and more than 9000 cases involved children under the age of 15.
“So there’s a lot of errors happening in homes,” Ms Thomson says.
Poisonings in baby boys up to 12 months old rose 23.2% per year, although the overall number of infant poisonings was small.
Poisoning rates in female adolescents and children rose by 20.4% and 18.5% per year respectively on average.
Females had over 2.6 times higher odds of intentional poisoning than males, and adolescents aged 15–19 years had 2.4 times higher odds than adults, which reflects broader poisoning trends, Ms Thomson says.
“The biggest group who is self-harming with medications and other substances is adolescent females, and they’re self-harming at increasing rates,” she says.
“And what we know from other research is that people use what they’ve got. So if you’ve got somebody who’s prescribed an ADHD medication, then that’s what they will take for self-harm. Whereas if they have lots of paracetamol in the house, they’ll take all the paracetamol.”
Dr Joynson points out that ADHD is associated with increased suicide risk and up to 84% of patients will have one or more co-existing mental health conditions.
“And because impulse control is one of the executive functions, if an ADHD patient is struggling, they might impulsively take extra of their medications—whether they meant to do it as an actual suicide attempt or not.”
Adolescents with ADHD are particularly vulnerable, he adds, noting their emotional regulation skills lag about 30% behind their chronological age.
“So intellectually they may be keeping up with their studies, but they can really struggle to regulate their emotions—especially big emotions like anxiety, depression, relationship breakdowns. And that’s where ADHD patients will do impulsive behaviours, or their mental health gets quite bad and they might impulsively do something like overdose on their medication.”
Dr Joynson says education and monitoring are key when prescribing ADHD medications.
“When used appropriately for the right condition at the right time, the medications can be very safe and very effective. We just need to make sure we’re educating patients that these medications do come with risks and side effects, so we need to make sure we’re managing and monitoring them appropriately.”
He suggests counselling parents to store medications where children cannot access them and to check they are giving the right dose.
ADHD often runs in families, he adds, and parents with the condition may need a system to help them remember whether they’ve given the medication or not. For example, they can fill a dosette box with the child’s pills so they can see if they’ve already dispensed a dose.
Ms Thomson suggests keeping medications for each child in different coloured baskets to help reduce mix-ups, and having a system to document that the medication has been given. For example, parents could keep a notepad they mark off or send each other a text after giving each dose.
“There are lots of different approaches and it’s just about exploring what’s practical for that family,” she says.
GPs may want to consider prescribing smaller quantities for patients at risk of self-harm so they have access to fewer tablets at any time, she adds.
You can call the Poisons Information Centre on 13 11 26 from anywhere in Australia 24/7 with any questions about how to manage a patient.
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