An ongoing shortage of New Zealand's most common antidepressant during the Covid-19 lockdown has one psychiatrist urging patients to consult a doctor before switching medication.

The drug fluoxetine, also known as Prozac, has been in limited supply since late February when Arrow-Fluoxetine tablets were moved to monthly dispensing instead of three months.

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The capsules of the same drug were reduced to monthly dispensing in November last year.

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That Teva brand medication was supposed to be phased out by April 1, in favour of the new funded drug Fluox, by Mylan.

Pharmac director of operations Lisa Williams said the Fluox brand was now not available because of manufacturing delays.

"To maintain continuity of supply, Mylan asked Teva to continue to supply Arrow-Fluoxetine until Mylan's manufacturing issues are resolved."

But there is also a shortage in Arrow-Fluoxetine and the latest delivery in March is now overdue because Covid-19 has affected the shipment.

"The supplier is working to get the stock air freighted to New Zealand as soon as possible and Pharmac is supporting them with logistics," Williams said.

"For people who are needing to fill their prescriptions for fluoxetine capsules now, pharmacists will be able to dispense them the tablet form of the same medicine."

Fluoxetine is used to treat depression, obsessive–compulsive disorder, bulimia nervosa, panic disorder and premenstrual dysphoric disorder.

More than 300,000 New Zealanders were prescribed antidepressants last year. At least 95,000 of them took fluoxetine.

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The Herald has seen a letter from one senior psychiatrist at a district health board recommending if patients cannot get supplies of fluoxetine they switch to another available antidepressant.

These include citalopram, escitalopram, paroxetine, or a different class of antidepressant such as venlafaxine or mirtazapine.

However, Tauranga psychiatrist Dr Caleb Armstrong urged against switching without consultation with a prescriber because those antidepressants had much greater withdrawal complications than fluoxetine.

"It usually takes four to six weeks of not taking fluoxetine or taking it at a lower dose before people get into major difficulties with relapse into low mood or anxiety or obsessive compulsive disorder.

"If this supply problem is fixed in the next couple of weeks there shouldn't be too many difficulties for people."

But more than a few weeks without the medication would mean a number of people will relapse into mental illness "and that's a concern".

Armstrong said switching to the other antidepressants could present longer-term problems if they also became in short supply as the global coronavirus pandemic continues.

Complicating that was the fact five weeks was needed between ending one antidepressant and beginning another, to prevent serotonin syndrome.

Symptoms include high temperature, agitation, increased reflexes, tremor, sweating, dilated pupils, and diarrhoea.

"It's not just stop taking fluoxetine one day and start taking another drug the next day - you'll get sick."

He said he did not want people to "freak out" about the shortage.

"Most people won't go into immediate withdraw state. Most people will not be greatly harmed. There are other drugs where that wouldn't be the case."

Citalopram, escitalopram and paroxetine have a significant withdrawal syndrome, Armstrong said.

"I would be significantly more concerned about shortages in those drugs because of that potential for withdrawal.

"For some people it may be more sensible for them to hang on and see what happens to the supply of fluoxetine than jump across to something else that could in turn have supply problems.

"The seriousness of this problem really depends on how long the supply crunch goes on for."

Both of the fluoxetine brand drugs were made in India.

A psychiatrist warns not to
A psychiatrist warns not to "freak out" over a shortage in supply of antidepressant drug fluoxetine, also known as Prozac. Photo / Supplied

Covid-19 had exposed the fragile interdependencies in the international pharmaceutical market.

China, where the outbreak began, is the world's largest supplier of bulk drugs.

India, which is a leading exporter of generic drugs, depends on China for more than two-thirds of its bulk drug needs.

Mental health advocate Jane Stevens said she was concerned about the shortage at a time when the country was in lockdown.

"It's worrying for people and certainly people don't need extra anxiety on top of what they're already facing," Stevens said.

She was also worried about the physical impact a brand switch could have on patients.

In October last year Pharmac changed its generic brand of epilepsy medication Lamotrigine to Logem, which was followed by several deaths.

At least five deaths were linked to the Logem brand sparking the Chief Coroner to launch and inquiry and a backdown from Pharmac.

Covid19.govt.nz: The Government's official Covid-19 advisory website