A man poisoned with cyanide at work and another who died after drinking beers laced with arsenic are being reviewed by South Australian cold case police.

Brian Underwood, 46, a father of two, was originally believed to have died from a heart attack in 1991 after he collapsed at work in Adelaide.

But a police investigation into another matter led to further post-mortem testing which identified that he had died from a lethal dose of potassium cyanide.

As part of the unrelated investigation, a woman who had worked with Mr Underwood was found in possession of a forged receipt for cyanide.

"She has been spoken to police about this matter and is a person of interest, but police are keeping an open mind about this matter," Detective Brevet Sergeant Newbury said on Sunday.

"As a result of testing we know that someone had deliberately interfered with Mr Underwood's stuff at work to deliberately poison him - he was targeted by someone who had access to his workplace."

The second case dates back to 1966, when Clarence Harry Greenwood, 40, drank beers laced with arsenic.

The beers were bought from a pub in Adelaide's western suburbs and Mr Greenwood drank them and was sick and found dead in his bed the next morning.

Police are trying to track down Mr Greenwood's nephew, Daniel Greenwood, and believe he may now be the only person still alive with "valuable information".

"We believe family frictions may have played a part in this murder," Detective Brevet Sergeant Erin Vanderwoud said.

""We don't know where Daniel Greenwood (now aged 66) is living, but he may have held the key to this."

Police say Daniel Greenwood was known to be working in the mining industry in Western Australia and are urging him to get in touch.

The two cases both have a $200,000 reward for information.

AAP

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