Requires main game only.
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Mod version 1.0
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Short story very short:
1. If you take Slasher after Psycho (has been taken), only the +25% Dam. Resist. bonus will be applied.
2. If you take Psycho after Slasher (has been taken), the +25% Dam. bonus will not be applied and you wasted a good chem.
3. On top of that (1 and 2), if Slasher expires, the +25% Dam. bonus (of Psycho) will disappear, even if Psycho is still active.
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This fix fixes that.
If both chems are taken, a Damage Bonus of +50% will be applied (base weapon damage). The +25% Dam. bonus (of Psycho) will no longer disappear, if Slasher expires and Psycho is still active (vice-versa is available too).
Works retroactively, if you loaded a savegame with Psycho active and Slasher expired.
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See the screenshots. I mean it.

Note:
You do not need this fix, if you use "Perks Day Tripper and Chemist Survivalist Style Fix" (https://www.nexusmods.com/newvegas/mods/66107).
Already included in version 1.4.

Have fun.

Credits:
Bethesda/Obsidian.
Me. Always.
The last sip of Bourbon taken from my trusted glass.

Many thanks to:
Nexus - for allowing us increased damage when using chems. In virtual life, that is.