Too big an intake could lead to diarrhoea and suppression of certain parts of the immune system, Dr Carine Smith of the university's department of physiological sciences said on Monday.
Sceletium tortuosum - traditionally also known as kougoed (literally, "something to chew") or kanna - is a succulent-like ground cover found largely in the Western Cape.
The Khoikhoi apparently chewed or smoked kougoed as a sedative with anxiety-alleviating properties and as a light anaesthetic for toothache.
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Smith, who has researched the plant's effect on laboratory rats, said this effect could be due to an alkaloid in the plant's leaves.
"So much of the information and so many claims about the benefits of kougoed that one finds on the internet and in books have never been tested scientifically with the appropriate controls," she said.
"They're based simply on untested tales and anecdotes.
"As in the case of any product with so-called healing properties, it's important to prove scientifically that people really benefit from it and that it's not just the placebo effect at play."
Smith's studies showed that dried kougoed had a limited positive effect on anxiety when taken in a low daily dose of about 5mg per kilogram of body mass.
What concerned her, however, were the numerous negative effects that she had observed.
When rats were given an increased dose of kougoed - 20mg - they showed signs of inflammation, diarrhoea and other forms of irritation of the alimentary canal.
The immune system could also be suppressed by increased intake.
"It's absolutely essential that more research be done to determine the optimal therapeutic dose for kougoed and other indigenous products," she said.
"There's a fine line between what's therapeutically good and what is, in fact, detrimental."
Smith said too few scientific tests were generally done to determine suitable doses for natural herbal products.