TWELVE women are among 17 prisoners who have had their sentences increased after pleading guilty to using ya bah inside crowded Phuket Prison.

The 17 Thais failed a regular urine test on December 21 and pleaded guilty when they appeared in court on Phuket on February 24.

Phuket Prison officials believe the drug was thrown over the walls of the jail inside a bar of soap - and that it ended up in the hands of people it wasn't intended for. The drug was probably burned on foil and inhaled.

All of the accused prisoners pleaded guilty and were sentenced to an extra 12 months, reduced to six months because of the guilty pleas. Visitor rights have been suspended for three months.

About 70 percent of the prisoners are serving sentences involving drugs. Phuket Prison is overcrowded, containing 1200 men and 170 women confined to a rapidly-ageing facility that was built to hold about 750.

Forty prisoners were sent to Songkla two weeks ago to relieve the stress but the crowding at present exceeds levels of 2009 and 2010.

About 15 staff care for inmates at any one time. Four British citizens are among the small group of expat prisoners.