They can cause gastric irritation in some cats, but in general are safe and effective. (felinefriendlycare.com)
The highest concentration is in the plum pit, but there is also a substantial enough amount in the foliage and roots to cause gastric irritation and possible respiratory distress if eaten. (wagwalking.com)
However, if gastric irritation persists alternative treatment may be necessary. (icatcare.org)