Police in Colorado believe a dentist killed his wife by lacing her pre-workout protein shakes with arsenic and cyanide, so he could be with a fellow dentist he was having an affair with.

James Craig, 45, was arrested on suspicion of first-degree murder on Sunday after his wife was taken off life support.

Craig’s wife Angela, 43, had visited hospital three times in the last month, with doctors unable to find what was wrong with her.

Police in the Denver suburb of Aurora began investigating the dentist after his dental practice partner and friend told a nurse that Craig had ordered potassium cyanide even though it was not needed for their work.

According to an arrest warrant laying out evidence gathered by investigators, Craig searched questions on Google such as “is arsenic detectable in autopsy?”.

Police believe Craig put arsenic in one of his wife’s protein shakes he routinely made for her.

After she survived, he ordered the potassium cyanide and told an office manager not to open that package.

Another practice employee did open the package, however, leading to it being reported to authorities.

Investigators believe that when his wife was in hospital, Craig was meeting with a fellow dentist who flew in to visit him regularly.

Craig’s friend who discovered the potassium cyanide also told investigators that Craig was on the verge of bankruptcy and had been having problems in his marriage.

Angela Craig’s sister, Toni Kofoed, told police that Craig had drugged his wife about five years ago with an unknown substance because he said he planned to kill himself and did not want her to be able to save him.

Ms Kofoed believes that incident is what the dentist referenced in a series of texts between Angela and James Craig when she first fell ill.

According to the arrest affidavit, James Craig wrote: “Given our history I know that must be triggering. Just for the record, I didn’t drug you. I am super worried though.”

Two days before Angela was declared brain-dead, the woman police say he was having an affair with wrote Craig an email expressing sympathy about what he was going through.

However, she said she did not think it was right for her to mix with those who were mourning Angela because she did not want to “conceal what I feel for you”.

The dentist is scheduled to appear in court on Thursday.