I looked into it further and it seems "Class 1 solvents such as benzene should not be employed in the manufacture of drug substances, excipients, or drug produc

I looked into it further and it seems "Class 1 solvents such as benzene should not be employed in the manufacture of drug substances, excipients, or drug products because of their unacceptable toxicity. The guidance notes that if benzene use is unavoidable to produce a drug product with a significant therapeutic advance, then its levels should be restricted to 2 parts per million (ppm), unless otherwise justified."