pmc.ncbi.nlm.nih.gov/articl...
This is from 2013, but still relevant. Jumping straight to conclusions it provides reasons why B1 is not easily absorbed orally...
...However, gastrointestinal dysfunction is common in patients with PD, and it potentially affects the therapeutic intervention 140. Gastric emptying has been reported to be frequently delayed in patients with PD 141. Decreased nonmediated uptake across the enterocyte brush border membrane was demonstrated in patients with PD 142. In addition, the intestinal absorption of thiamine is sufficient in young people but may be reduced with age 143. Parental administration of thiamine may be suitable for patients with PD 14....
Parental=injectable.
Anyone is aware of a dairy free sublingual version?