Police are warning Christmas revellers to be alert after two women had their drinks spiked.

In separate incidents in Palmerston North over the past three weeks, the women drank small amounts of alcohol but suffered the effects of drink spiking.

Symptoms included feeling light-headed or dizzy, memory loss, numbness, feeling unwell and, in one case, being violently ill the next day.

"It only takes a second to drop a pill, some powder or a few drops of liquid into a drink," said Manawatu adult sexual assault squad head Detective Sergeant Gary Milligan. "The effects of these drugs can start in 10-20 minutes, making a person sleepy, dizzy, uninhibited, relaxed and open to suggestion. This is when women are at risk of rape and sexual assault."

Mr Milligan said the incidents were relatively unusual, although it was likely that some cases of drink spiking went unreported.

Both women, in their early 20s, had been at central city bars before they fell ill. One had reported to the police relatively quickly, and officers were still following up her case, including doing drug tests.

The other was too late for drug tests.

Mr Milligan said spiking cases could be difficult to investigate as the women invariably had no idea who had done it, due to the effects of the drugs.

He said a prevention approach was the best one to take, and added it wasn't just drug spiking that could lead to rape.

"Alcohol is the most common drug used to assist in sexual assault as it can affect actions and reactions."

Police asked women to be on the lookout for drink spiking and take precautions at a bar, club or party:

• Buy own drinks
• Be wary of accepting drinks from strangers or people you don't know well
• Avoid sharing drinks
• Watch your drink being poured or can/bottle being opened at the bar
• Keep an eye on your and your friends' drinks when dancing, talking or going to the toilet
• If your drink looks cloudy, changes colour, doesn't taste right, then don't drink it
• If feeling drowsy, suddenly "out of it" don't just hope it will pass
• Have a friend or someone you trust take you to a safe place
• Watch your friends, look after and watch out for each other

Anyone with information relating to drink spiking is encouraged to phone Crimestoppers 0800 555 111.