could be from 10,000 things

without knowing what foods he eats,
current state of colon / liver/ toxicities etc..
it could be from an endless group of potentials.

this is a question i would definitely refer you to ask
at the Ask Andreas Forum .
Watch for when his answers get close to the top again
(his posting name is Andmor)
then when he gets near top ,
he reopens the forum for new posts,
to ensure that everyone
gets a personal response
from him as time allows.

In the meantime this is a good start spot to browse:

http://ener-chi.com/num_one.htm

good mouth rinse you might consider,
is sesame oil swishes,
clove oil,
pau d arco herbal decoction,
good high carvocal oil of oregano,
and /or olive leaf oil diluted s well.
some thoughts along that subject.

hope this helps you .
also read the links I posted about
toxicities in many food products.
Love,
Ami Joi Benton