However, the development of a new slow-release ketamine tablet may soon change that.
University of Otago Hazel Buckland chair in psychological medicine Prof Paul Glue said doctors usually used the anaesthetic drug as an effective means of sedation and pain relief.
Ketamine is also given to treatment-resistant patients via injection or nasal spray, leaving them with major side effects.
"It can make patients feel very spaced out, very sleepy. They can’t walk for about half an hour.
"That’s why you have to have it in a clinic.
"It’s quite a burden on resources for patients and the health system."
Prof Glue said depression was considered a "high prevalence disorder" because 15% of the population would be depressed at some point in their lives, and about a third of those people would have treatment-resistant depression.
He said the researchers had worked with Douglas Pharmaceuticals to create, and trial, ketamine in an extended-release tablet form.
A study of the new tablet involved 168 anti-depressant-resistant adults, who either took a range of oral doses of ketamine or a placebo for 12 weeks.
Prof Glue said the highest dose of ketamine — 180mg — showed significant improvement in depressive symptoms, compared with patients who received the placebo.
"The results were very promising in the sense that patients didn’t get the typical side effects that you get from injected ketamine or from the nasal spray.
"With this, it offers the possibility that patients could just be given a bottle of tablets to take at home."
It would potentially make treatment a much cheaper and more convenient option for these patients, compared with weekly clinic visits for ketamine injections or nasal sprays, he said.
However, one of the major concerns was that the new tablet could be abused by recreational drug users.
Prof Glue said having it in a slow-release tablet form reduced the risk of abuse because the manufacturing process made them difficult to manipulate.
"People who are abusing ketamine, or a lot of other recreational drugs, are getting very high blood levels right away.
"But with this drug, because it releases slowly over about 10 hours, it’s just not physically possible to get those high peak levels."
Douglas Pharmaceuticals was now seeking the interest of partners to complete registrational clinical trials and prepare for commercialisation of the tablets, and Prof Glue believed they could be available to the public from pharmacies in the next two years.
