This mod is no longer maintained, a replacement can be found here: 
https://www.nexusmods.com/witcher3/mods/7414

About:

This is a small WIP mod that I made because I was bothered by the fact that Geralt can just use alcohol to refill all of his potions, including oils and bombs.
The mod tries to make alchemy more realistic and like in the previous games, but without changing too much about the vanilla behavior. I know mods like this already exist, but I mainly made this for personal use. Some things are probably not properly balanced yet, so I appreciate any feedback.

Features and Changes:

Alchemy items behave like any other item:

• Once you use them, they are gone and you have to make them again.
• The amount of each potion, bombs, oils etc. that you can have is no longer limited, you can craft as many items as you need.
• Alchemy items can be stored in the stash and sold to merchants.
  

Reduced cost:

• Brewing no longer needs as many ingredients as before.
• Only renewable ingredients are needed.
• Merchants sell a lot more herbs (not more kinds of herbs, but a higher quantity of each).
• To make mutagen potions, you still need to have the according mutagens but they are not consumed when making the potion. (Be sure to always keep one of each mutagen!)
• When crafting alchemic substances (Nigredo, Albedo etc.) you now get 5 substances each time.
• You get back an empty bottle when using alcohol to make substances.
  

Improved Stats:

• Increased potion duration by 400%
• Increased mutagen potion duration by 100%
• Increased oil uses by 100%
  

Changable bases:

• The base items (strong alcohols for potions, fats for oils, powders for bombs) are no longer fixed for each recipe but can be swapped out for other items of the same type.
• To do so you have to select the item and press X (R2 on gamepad), which opens a dialogue where you can select the item you want to use. You can also use the arrow keys (dpad_up/dpad_down on gamepad) to scroll through the items.
  

Manticore armor and cockatrice decoction:

• The Cockatrice decoction and the second-tier bonus of the Manticore armor both previously increased the maximung amout of potions you could carry by 1. Since the number is no longer limited, these features are useless and they now both give a +50% chance of acquiring additional herbs. This perk is already in the game but it was never used because there was no point in collecting too many herbs anyways.
• This means that you will always get an additional herb when wearing the full Manticore armor and having the Cockatrice decoction effect.
  

Laboratory in Corvo Bianco:

• Like the Manticore armor and the Cockatrice decoction, the laboratory in/under Corco Bianco previously increased the maximum amount of alchemy items by 1. Instead of this, it now opens the alchemy screen when being used.
• Making potions, bombs or oils in the laboratory is a lot more efficient and you now get two instead of just one item when you make something in the laboratory.
  

Other Changes:

• You can directly navigate to the recipe of an ingredient using C (R1 on gamepad)
• Items in the alchemy menu are now primarily sorted by their quality (higher quality items are on top) instead of their name.
• Mutagens now have their own category and are no longer in the "Alchemy Substances" category.
• Before brewing something, there is now a dialogue, where you can choose the amount of items you want to make.
  

Installation:

• Unzip the files into your "mods" folder (create one if it doesn't exist) found in your Witcher 3 main game directory or use the mod manager. Use ScriptMerger to resolve script issues.
• (Optional: Use the command "betteralchemy_setup" ingame. It will replace all mutagen potions with the source mutagens and update the Manticore effect which will otherwise only update if you unequip and equip a piece of the armor. This command can only be used once.)
  

Recommended Mods:

• FriendlyHUD: Merge with ScriptMerger
• More Quick Slots and Quick Inventory: Use the version for MoreQuickSlots of this mod, merge with ScriptMerger (select A)
• Preparations: Use the version for Preparations of this mod, merge with ScriptMerger