Long-term Senna use OK in small doses?

I wrote to Beachwood Canyon Naturally asking why their parasite formula (PC-123) has Senna (its last and least ingredient), and whether they thought it was harmful. Any opinions on this response?

"Senna is a dual function herb. In large doses it is a stimulant laxative,
and your body can become dependent upon any laxative. In small
doses it tones the colon and works synergistically with several of the
other herbs in the formula. Small levels can be taken safely over a
long period. In this case it is a very small dose and is there to tone
the colon and to aid the other herbs."