DURHAM — It is “reckless” for the Region of Durham to supplement its drinking water supply with fluoride, an expert witness has testified.

“The practice should stop immediately,” Dr. Paul Connett said Friday. “I’d stop it tomorrow if I could.”

Dr. Connett, who has co-authored a book on the issue, was testifying in support of a suit filed in small claims court by Whitby resident Russell Brown. He’s suing the Region to recover the cost of a filtration system he uses to remove fluoride from his drinking water.

While the amount of the claim — about $900 — is relatively low, the issue it represents is significantly larger: Mr. Brown’s ultimate goal is to coax the Region to stop adding a fluoride compound that he says is a byproduct of fertilizer production to the community’s drinking water.

“It’s to make them accountable,” the 45-year-old father of two said outside an Oshawa courtroom. “This is a toxic waste substance — it causes harm.”

The Region rejects the assertions in Mr. Brown’s suit, and is contesting his claim for damages. On Friday, Regional lawyer Rajeshree Sanichara suggested Dr. Connett is making unfounded claims about Durham’s water without having data to back those claims up. Dr. Connett said fluoride affects IQ levels in children and can, when ingested in excess, cause damage to teeth and bones.

“I put it to you that you can’t say the Region of Durham’s water is not safe if you haven’t tested it,” Ms. Sanichara said.

But Dr. Connett insisted the benefits of fluoridation — it’s added to combat tooth decay — pale in comparison to the health risks.

“If you want fluoride, you can brush it on your teeth,” he said. “There is no reason to swallow fluoride.”

Mr. Brown and his supporters, some of whom are members of Free Durham From Fluoride, came armed with statistics and legislation they say support their claim that Durham’s fluoridation program is in contravention of the Safe Drinking Water Act.

What they lacked in polish — Mr. Brown was represented by Robert Fleming, a former police officer who stepped in at the last moment to conduct the case — they made up for with earnestness. Deputy Judge W. Mark Burch repeatedly and patiently reminded Mr. Fleming to stick to the question at hand: Has Mr. Brown made out his case that he has suffered damages for which he ought to be compensated?

The Region has yet to present reply evidence. The trial resumes in May.