The vials of venom found throughout Britannia have advantages and side-effects much like those other characters warn of; the venom will affect your party's attributes by raising their Strength, Dexterity and Intelligence.
Each vial will raise all attributes by five and then subtract them by six (e.g. an 18/18/18 Avatar will become 17/17/17 at the end of the hour). The cap is 30/30/30, so there's no need for a highly leveled character (or a post-Forge of Virtue Avatar) taking venom.
After one in-game hour the poison will wear off; you will hear a noise somewhat like opening an inventory window and some of your party will comment that the drug user doesn't look too good. Their attributes will have come down to their previous value, but are now one point less per vial taken. Note that in the DOS original the Avatar's attributes will sometimes not come down at all.
However if you take five simultaneous shots that person's attributes will immediately plummet to 1/1/1 as soon as the venom wears off. This can also be achieved by taking a large number of simultaneous doses to eventually whittle them down to this level. It must be five or more; four simultaneous doses will only result in 6/6/6.
When running under Exult it won't go any lower than this, but the original game had a bug where the attributes would roll over if they reached zero. If you are using Exult none of the following will work, you'll still only get a 1/1/1-stat character.
If you take another 5-vial overdose when the character's now-normal attributes are already low, his/her strength will be forced below zero and thus end up somewhere around 200-255, depending on the attributes he/she had to begin with and how many doses above five were taken once they reached the 1/1/1 margin. This is far more than the 30/30/30 cap training stops at, and even makes a 60/30/30 post-Forge of Virtue Avatar look like a weakling. Other attributes will be affected as well, but strength is the main one. Because of the attribute increase, the character's health points will leap to somewhere around 100-125.
Despite these positive results, drug use does have a downside; all healers will comment that the drug user is "badly wounded", even if the character isn't poisoned and has full health. After a healing, health points will lower to a negative value that is almost the exact opposite of their points before healing (for example 130 becomes -110). The game doesn't class them as dead, but they aren't living either; because they are at an in-between attributee they will be completely immobile. Moving around without them onscreen will cause immense slowdown.
Because of the complexity of fixing them once in this undead attributee it is best to not use healers on them at all, instead rely on amulets, bandages, potions, wells, etc.