The most commonly used solvent for thymoquinone would be dimethyl sulfoxide, whereby sometimes olive oil may also be added. You can have a read on the following papers.
Article The Effect of Route of Administration in Thymoquinone Toxici...
Article Evaluation of the Effect of Thymoquinone Treatment on Wound ...
Article Oral and intraperitoneal LD50 of thymoquinone, an active pri...