A welathy businessman man lured a young real estate agent to his home and laced her food with the male sexual enhancement drug Viagra, a court has been told.

It is alleged the well-known man lured the saleswoman, who was more than 30 years his junior, to lunch at his Mid-Canterbury homestead to discuss a lucrative property listing.

The woman, aged in her early 20s, told the court yesterday she found her meal of microwaved spaghetti bolognese was tainted with pill fragments that made her feel out of control.

She said that while she was feeling affected, the businessman lunged at her and tried to kiss her. She pushed him away.

The woman was adamant she had done "nothing whatsoever" to indicate she desired his attention and said the man knew she had a fiance, Christchurch District Court heard.

The businessman, who has interim name suppression, denies charges of indecent assault and poisoning with intent.

In evidence before a jury of seven men and five women today, Evan James Begg, a professor of clinical pharmacology, told the court Viagra was a drug used to enhance male sexual performance and was prescribed for erectile dysfunction in men.

Professor Begg said Viagra's local effect was to engorge blood vessels in the genitals of men "to enhance what you've already got". It had a similar effect on women's genitals, but had no corresponding effect on their libido or desire.

Side effects of the drug sildenafil, which was marketed as Viagra, included headache, dizziness, nausea, light-headedness and in higher doses changes in colour vision.

He told crown prosecutor Craig Ruane that crushing a tablet would make its contents more readily available for absorption, but the effects could be delayed if taken with food.

Prof Begg said a slightly built person would be likely to have enhanced effects, depending on the dosage. He agreed that the description "woozy" would fit with the drug's known side effects.

Cross-examined by defence lawyer Jonathan Eaton, Prof Begg said Viagra had never been a drug for women, but reports in the "lay press" and on the internet suggested it had been used by women.

"It's not prescribed for women. It's not a women's drug," he said.

He agreed it was "fair comment" that giving a woman Viagra was unlikely to enhance the chances of sex.

Prof Begg said he believed the case was the first before the court involving Viagra being used in an alleged sexual assault. It was not known as a drug for "stupefying" or relaxing women and was not known as a "date-rape" drug.

The trial, before Judge Phil Moran, is expected to conclude tomorrow.