I wanna have venom glands, maybe be able to gleek it straight into someone's eyes but mostly have teeth built for injection. While the intended route of administration is intramuscular injection, mucosal absorption or ingestion are also commonly used, and intravenous injection is rarer but still used (this requires a very low dose!). The venom's all one substance, but has different effects based on dose.
At low doses (5-80ΞΌg), it's a CNS stimulant. It's primarily a dopaminergic (both an agonist and a reuptake inhibitor), but also has some lesser action with serotonin receptors. It's almost exclusively active at the CNS, there's a little PNS activation but far less than comparable drugs. There is a substantial analgesic effect as well. It's deeply euphoric but is quite safe at this low dose; no risk of neurotoxicity like with methamphetamine or high-dose amphetamine, and while it's rather addictive the withdrawal symptoms are comparatively mild. It's worth noting that, at extremely low doses, the stimulant effect can be overpowered by the analgesic effect of the drug, and the drug itself can act as a CNS depressant.
At moderate doses(80-450ΞΌg), the stimulant and euphoric effects are more pronounced, but there are diminishing returns. Instead, as the dose is increased there is an aphrodisiac effect on the target, subtle at first (this can be used for persuasion, or alongside more blatant forms of hypnosis). As the dose increases and the effect strengthens, the target's suggestibility (particularly to carnal pleasures) increases dramatically; at the upper end of this range, the target is effectively limited to begging for sex, or performing alternate sexual acts as suggested. The senses (particularly tactile and olfactory/gustatory) are sharply enhanced, corresponding to dose. In addition to the steady overproduction of sexual fluids (mass production of cum as well as constantly leaking pre; mass production of vaginal lubrication), the penis/clitoris and nipples will remain firmly erect for the duration of the drug's effects and orifices (obviously including vagina, anus, and mouth/throat, but also the urethra, nipples, ears, nostrils, tear ducts, and navel, along with any ostomy or other substantial wounds) will be more elastic and more easily relaxed. The increased action at serotonin receptors will effectively halt motility in the stomach through the small intestine, though the pharynx and large intestine will be quite active. In addition to sexual fluids, the body will rapidly produce red blood cells, urine, and milk (if lactating). Combined with the strong stimulant effect of the drug, the aphrodisiac effect can produce an OCD-like obsession in the target, almost exclusively focused on sex or sensuality. At the upper end of this window, the analgesic effect begins to taper off, likely due to the overall increase in sensation. The target usually doesn't mind this - at this dose, they are being driven entirely by their immediate lust and will gladly accept greater pain as a strong source of stimulation. They can tell that they are experiencing pain, but even those typically averse to pain play find the sensation of pain pleasurable and become eager masochists, with no concern for the long-term consequences of whatever inflicts that pain.
At high doses (450ΞΌg-1mg), these same effects continue to build, the target being immensely stimulated and orgasming unpredictably to the sensation of clothing, minor air currents, and even the floor underfoot. Soon after analgesia gives way to masochistic please, the target beging to experience growing paralysis. The mechanism is unclear, but is believed to have something to do with serotonergic pathways? Fortunately for the target, the loss of fine motor skills for skillful masturbation is accompanied by even greater increases in sensitivity, giving them a hair-trigger easily set off by their desperate pawing at their most sensitive parts.
At extreme doses (1-2.5mg), the nature of the drug as a venom becomes more apparent as the effects become more lasting damage. The target is overstimulated, uncontrollably aroused with no way to resolve this problem - masturbation and repeated orgasm only increase the effect. The sensitivity of the target continues to increase, causing unbearable agony. It is not possible to get good data from targets mid-experience, but survivors variously claim sensitivity so high that slight air movement was like razor blades, or that every sensation was pleasant but that this further fueled the aphrodisiac effect to the point of torment. As the effect increases, the target loses motor control (beyond the usual effects of CNS stimulants as seen with doses below this window) and is unable to do much but try to vocalize (with dropping clarity) a need for sexual release. At the upper half of this window, the stimulant effect becomes more powerful and euphoria increases rapidly - this is due to an effect similar to methamphetamine, where neurons are burst open to release a shower of dopamine. Simultaneously with the growing paralysis of the body, cholinergic activity begins to occur as the drug is metabolized in neuron explosion and as stored acetylcholine is released. The target is reduced to a desperate, twitching mess, unable to do anything but moan and hope it is raped.
2.5mg is the human LD50 - that is, half of human targets will die from a dose of 2.5mg. The destruction of the nervous system and signal failure of the brainstorm causes the heart to fail under the strain, combined with the increased physical demands resulting from cholinergic effects. Above 5mg, this happens quite rapidly. Failing this death, paralysis will eventually grow to encompass the stoppage of the target's lungs or heart, with consequent brain death. This takes considerably longer though, and is not usually the cause of death.