It was pharmacologically active.

The arecoline was getting into the body.

It was acting on brain receptors.

It was producing effects.

The existence of this pharmacological activity is what makes the ancient dental wear pattern interpretable as drug use rather than simply as an unusual dietary habit or stress behavior.

Combined with the chemical detection of arecoline in the dental tissue itself, the experimental demonstration closes the explanatory loop: the behavior produced the drug; the drug was in the teeth; the behavior continued despite the damage it caused.

Why Dental Tissue Preserves Chemicals So Well