For some bizarre reason, on one of my playthroughs Sveta has become permanently stoned. She got raped, and the rapists fed her amphetamines and aphrodisiac gum, and since then for the last week she's been continuously stoned and her mood hasn't dipped below 100 once. There's no indication of her being under the influence of any narcotics, but her mood stays at 100%, even when she does all the things that would normally cause it to go down. I even tried getting her totally drunk, by visiting Vasily, Dan and Vitek, and despite downing over 10 drinks, apart from slightly reducing her health, she didn't get slaughtered as she normally would. And the following day she didn't have a hangover. So now she seems to have developed an enormous tolerance to alcohol, despite only getting slightly drunk on two previous occasions.
Don't get me wrong, permanently being at 100% mood is brilliant. But how could this happen?