By Eric Brand

Xi Xin is a medicinal that tends to have limited availability due to concerns about aristolochic acid (AA) content. Aristolochic acid is a known nephrotoxin and carcinogen, and botanical products that contain it are strictly controlled for sale in many countries around the world (both in East Asia and the West). However, many practitioners remain unclear about the situation with AA and Xi Xin, particularly with concern to the plant parts used in medicine.

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The root of Xi Xin is very low in AA, but the aerial portions (esp.
the leaves) contain significant levels of AA. Consequently, the
pharmacopoeias of Taiwan, mainland China, and Hong Kong all now only
specify the root and rhizome as the official drug. If we look at
older editions of the PRC Pharmacopoeia, we can see that the whole
herb is listed in the 1995 edition, but by the 2005 and 2010 editions
only the root and rhizome are listed. This reflects the discovery of
the harmful effects of AA and the trend to eliminate the clinical use
of the aerial portion of the plant. Of course, it takes time for the
message to get out, so it is not uncommon to still see the whole herb
used in both China and the West.

Many Chinese materia medica scholars believe that only the roots of Xi Xin were used in ancient times, which is supported by evidence from the ancient materia medica
Ming Yi Bie Lu. The name Xi Xin means "thin acrid," which is thought
to be a reference to the thin shape of the roots and their strong
acrid taste.

The root and rhizome of Xi Xin contain a medically negligible amount
of AA, but samples of the whole root often still test positive for the
presence of AA with a highly sensitive test. This makes even the
roots technically illegal for use as a food or dietary supplement in
the U.S., but the roots are permitted for internal use in China, Taiwan,
and Hong Kong. When the roots are used in a decoction (including
granules made by decoction), the poor solubility of AA makes the
resulting solution extremely low in AA. In fact, some granule companies
use an FDA-approved testing method to demonstrate that there is no
detectable AA present in the granule product, which presumably allows it to be
legally used internally. (I think the detection limit used is 0.03 ppm if memory serves.) However, import of Xi Xin can still be problematic, as it often subject to a general customs hold if Xi Xin products are labeled for internal use. Given these factors, few suppliers choose to stock it so it remains of limited availability.

It is important to note that the traditional toxicity of Xi Xin, as well as the ancient saying "xin bu guo qian" (the dosage of xi xin should not exceed one qian, or 3.7
grams) is not necessarily related to AA, and even AA-safe preparations
of the roots should be used with proper attention to dosage and
pattern. In particular, dosage of the ground, crude medicinal should be carefully controlled.

For more on Xi Xin and AA, check out this excellent paper by Zhao Zhongzhen et al.