Which is the best moment give a Pokemon a raise stat item?

Effort Values (EVs)

Pokemon have hidden stats called 'Effort Values' (EVs) - one for each stat (HP, Attack, Defense, Speed, Special), with a value range of 0 - 65535 in Generation 1. These EVs are used to calculate the Pokemon's stats (the ones you can see on the Pokemon's summary screen). EVs can be gained by defeating other Pokemon in battle, or from using Vitamins (up to a point).

Any increase in EVs is not immediately factored in to a Pokemon's stats. A Pokemon's stats are only recalculated from three events:

• When the Pokemon levels up
• When the Pokemon evolves
• Using the Box Trick (storing, then retrieving a Pokemon from Bill's PC)

When to apply Vitamins

You should give Pokemon raise-stat items - also known as Vitamins - pretty much immediately. Do this before training them in battle, so you can take advantage of the maximum gain from the vitamins.

Each vitamin will apply 2560 Effort Values in the given stat, up to a limit of 25600. After you reach 25600 EVs in that stat, you'll need to train that Pokemon manually to increase the EVs beyond that limit.