This stuff ebbs and wanes in popularity, every 10 years or so it comes back into popular use; then fades away because of its intolerability, repeat.
As
@theotherguy mentioned, it's not absolutely necessary to use guaiacol as solvent, but it's almost universally used by UGLs for 1-Test ("DHB") to hold the solute in solution so that PIP complaints/negative feedback is diminished.
Guaiacol is a chemical irritant, a disinfectant, and cough expectorant. It is classified as a hazardous chemical.
According to the World Health Organization's International Agency for Research on cancer, guiaicol is probably carcinogenic to humans. From: IARC. Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans. Geneva: World Health Organization, International Agency for Research on Cancer, 1972-PRESENT. (Multivolume work). [link redacted],p. S7 177 (1987).
Adverse effects in humans include:
Neurotoxin - Acute solvent syndrome
Occupational hepatotoxin - Secondary hepatotoxins: the potential for toxic effect in the occupational setting is based on cases of poisoning by human ingestion or animal experimentation.
Occupational hepatotoxin - Secondary hepatotoxins: the potential for toxic effect in the occupational setting is based on cases of poisoning by human ingestion or animal experimentation.
Nephrotoxin - The chemical is potentially toxic to the kidneys in the occupational setting.
Dermatotoxin - Skin burns.
Its use orally in humans acutely induces gastrointestinal, behavioral, tremor, and other changes.
From what I have seen, 1-Testo seems to always lead to elevated C-reactive protein (CRP), whether directly due to its inflammatory or immunogenic effects starting in muscle tissue, or due to guaiacol, or both. CRP is a liver-secreted marker of systemic inflammation that has some predictive power in cardiovascular/thrombotic risk, and is especially dangerous when elevated along with Lp(a) and LDL.