In its pure form, chamazulene is considered non-toxic and non-irritating. However, oxidized chamazulene can become a skin sensitizer, so oxidation during storage should be avoided.
According to reference text Essential Oil Safety by Tisserand and Young, there is a hypothetical risk that chamazulene-containing oils could interact with certain medications. Chamazulene may inhibit the cytochrome P450 enzymes CYP1A3, CYP3A4, and CYP2D6 which metabolize some drugs.
In theory, this could cause higher blood levels of medications like antidepressants, calcium channel blockers, chemotherapy agents metabolized by these pathways. However, drug interactions with chamazulene have not been directly studied.
Those taking medications with narrow therapeutic windows that rely on these CYP enzymes should use caution when using chamazulene therapeutically until more data is available.
Monitoring medication side effects and blood levels is advised.