Some animals naturally secrete a substance called bufotenin/bufotenine:
Bufotenin (5-HO-DMT, N,N-dimethyltryptamine, bufotenine) is a tryptamine related to the neurotransmitter serotonin. It is an alkaloid found in the skin of some species of toads; in mushrooms, higher plants, and mammals.
In 2001, ethnobotanist Jonathan Ott published the results of a study in which he self-administered free base bufotenine via insufflation (5–100 mg), sublingually (50 mg), intrarectally (30 mg), orally (100 mg) and via vaporization (2–8 mg). Ott reported “visionary effects" of intranasal bufotenine and that the "visionary threshold dose" by this route was 40 mg, with smaller doses eliciting perceptibly psychoactive effects. He reported that "intranasal bufotenine is throughout quite physically relaxing; in no case was there facial rubescence, nor any discomfort nor disesteeming side effects".
At 100 mg, effects began within 5 minutes, peaked at 35–40 minutes, and lasted up to 90 minutes. Higher doses produced effects that were described as psychedelic, such as "swirling, colored patterns typical of tryptamines, tending toward the arabesque". Free base bufotenin taken sublingually was found to be identical to intranasal use. The potency, duration, and psychedelic action was the same. Ott found vaporized free base bufotenin active from 2–8 mg with 8 mg producing "ring-like, swirling, colored patterns with eyes closed". He noted that the visual effects of insufflated bufotenine were verified by one colleague, and those of vaporized bufotenine by several volunteers.
This substance is mostly known from amphibians, such as the Colorado river toad and the australian cane toad. It wouldn't be a stretch for a dragon to produce it as well.
As for methods of delivery, let's review the ways in which it has been tested on humans again:
(...) ethnobotanist Jonathan Ott published the results of a study in which he self-administered free base bufotenine via insufflation (5–100 mg), sublingually (50 mg), intrarectally (30 mg), orally (100 mg) and via vaporization (2–8 mg)...
- Insufflation and via vaporization: these could be done by a breath attack, in addition to the breath attack you already mentioned your dragon has;
- Orally, sublingually: This could be a defense against predators. Any animals that bites a dragon (or swallows, if they are big enough) will suffer from the venom's effects, and should quickly learn how to avoid the dragons. This is similar to what some butterfly species do, to avoid being predated by birds. Some die but most benefit from this kind of defense. Notice that some humans might lick/bite the dragons on purpose should the venom work this way;
- Intrarectally: decorum requires me to leave this completely up to your imagination. It should make for some really scary legends involving the dragons and why they should not be messed with, though.