Silver Serpent Venom

Silver Serpent venom is first heard of in Ultima VI, as the gargoyles milked it from silver serpents to give to their warriors as a strength-enhancing battle drug. Using the venom in this way was fatal to them. It appears in Ultima VI, Ultima VII and Ultima IX.

Description
More was learned of Silver Serpent Venom in the Age of Armageddon. While no silver serpents are seen in this period, there is quite a lot of venom available. One NPC mentions that he gets his venom through contacts in Buccaneer's Den. While it is never said that the venom is an illegal substance, the Britannian government is definitely trying to control the sale of it at this point in time. It is apparently legal to sell the venom to apothecaries (the Avatar can sell venom to Kessler in Britain), who have the facilities to study the substance.

In Ultima VII there also is an illicit side-trade of venom to the Britannian Mining Company, who used it to keep their gargoyles working longer hours.

Effects of Silver Serpent Venom
Silver serpent venom is a reagent that, when ingested in small doses, temporarily enhances one's physical strength, stamina and quickness, along with bringing a feeling of euphoria. After the effects wear off, the subject feels quite drained, compelling many users to seek another dose. Prolonged use in such a fashion will bring about a condition that deteriorates the skin, eventually causing it to rot away. Too great a dose or accumulation of doses is fatal, as the venom is a deadly poison.

Gargoyles have a greater resistance to the effects of the venom than humans, and must ingest more of it to get a beneficial effect; unfortunately, the adverse effects are brought on more swiftly as a result.

Silver Serpent Venom as a Reagent
Mage's dream, youth's bane. Once upon a time, the ability to gather this reagent was heralded as the beginning of a new era of magic craft. It was the epitome of high wizardry, but alas, as frequently happens, the promise paid but little. While direct doses of this reagent give a boost to strength, the permanent damage done to the body far outweighs any temporary advantage. Unfortunately, these adverse effects tend to carry over to the spells cast using this reagent.

Before his illness, Garok Al-Mat, a mage of the high mountains, was experimenting with the venom in conjunction with spider silk and giant bee pollen. His hope was to bind the qualities into an effective casting reagent for divination, but his work is now lost forever.

Silver Serpent Venom in Ultima IX
Venom is a main component of Serpentwyne, a potion that completely restores health and mana without any of the ill-effects of the venom. The pure venom however can also still be found or bought on Buccaneer's Den. Drinking it, the Avatar temporarily enters a berserker-strength phase (life bar is purple), only to get hurt once the effect of the venom dies down.

Trivia

 * Blades coated with serpent venom made an appearance in HoBLotH II where a non-deadly blow would lead to 10 seconds of invulnerability before dropping dead. This led to a tactic for rushing the flag which involved envenoming your own team members to make them immortal to break through the enemy line.
 * Dinking the venom before having sex with the prostitute Jade in Ultima IX results in her turning into a man when the Avatar wakes up.