Indra Ningsih, 26, admitted to police mixing the blood in a pot of vegetables she was cooking, and was subsequently charged with "administering poison or other destructive or noxious substances with intent to injure."

However, prosecutors decided the charge could not be sustained, after the government laboratory and doctors said menstrual blood is not toxic, although it may carry a virus.

A Department of Justice spokesman said following a police investigation, the department found it would be inappropriate to prosecute, and dropped the charge.

Commenting on the case, solicitor Wong Kwok-tung said one option would be a civil charge, but at worst this could only lead to a fine.

"The only thing the employer can do is to fire her immediately and to seek to ban her from working in Hong Kong," Wong said.

He said Kwun Tong Magistrates' Court could not rule on the voodoo implications of using menstrual blood in food.

The court earlier heard the maid was told to cook some vegetables at her employer's home in Tseung Kwan O on April 15. However, the employer, surnamed Mok, peeked through the kitchen door and saw Ningsih acting suspiciously.

Mok entered the kitchen and found the domestic helper throwing something into the trash bin. Mok then found a blood clot- like substance mixed with the vegetables and water in the cooking pan.

She later discovered a used sanitary napkin in the bin, and called police. Under police caution, Ningsih claimed Mok had been unhappy with her performance, constantly scolding her since she was hired in July 2008.

The maid admitted mixing the blood with the vegetables in the belief that it would make Mok "more amicable and less picky."

In December 2007, another Indonesian maid was jailed for three months under the same charge that Ningsih faced, after being convicted of adding urine to the drinking water of her employer.

Wong explained the charge in that case could have been sustained, since the water with urine may not have been boiled and consequently may have been poisonous.

"You cannot lay a criminal charge on a maid, even if she puts saliva and cockroach in soup, which is then cooked," Wong said.

He added it would be a different case if blood or a cockroach was found in a dish served at a restaurant, as this would be in breach of the Public Health and Municipal Services Ordinance.