A nationwide shortage of liquid morphine could see patients missing out on the pain medication for months

Pharmac is warning supply varies by location, and it's encouraging clinicians to look at alternative treatments where possible.

Previous manufacturer Pfizer closed its Australian factory last year - and the company that's taken over is unable to supply to New Zealand until June .

Pharmac says looking for an alternative supplier over the past two years has been challenge.

It says it's managed to secure two new suppliers, but it expects low stock and delivery delays to create gaps in the foreseeable future.