Sage's brew:

Sage's brew is basically a fantasy espresso. The way it's created can be determined by the DM. Coffee beans could be a rare import from a distant land where they're cheap and common. Maybe an alternative method of creation is distilling tea, which might be more common, but needs to be concentrated by a factor of 20. Or just say that it's a secret recipe of herbs held closely by sages and a small subset of nobles. I recommend saying that all of these are valid ways to create the brew, just like there are many ways to get caffeine into your system in the real world.

Bitter taste

Before letting the character drink the brew, ask the player how the character feels about bitter tasting food and drink.

Differences in taste

A dwarf, an elder, or someone who is more of a beggar than a chooser when it comes to food is probably used to bitter ale or has gotten used to finding many tastes palatable.

Young or spoiled characters are likely to have a sweet tooth and find bitter tastes repulsive.

If the character doesn't have a taste for bitter drink, make a DC 3 constitution check, failing the check means your character spits it out. You never have to make this check again. Anyone who knows that this check has to be made in advance gets advantage one it.

Diarrhea

Depending on your gut biome and chemistry, the brew may... disagree with you.

Make it a surprise

For extra fun, the DM should make this random - roll a d20 and make that the new 20. For example if you roll a 3, 13 though 20 and 1 through 3 will pass a DC10 check. If you roll a 16 then 6 through 16 will pass a DC10 check.

Make a DC 10 constitution check, failing the check means you don't have the stomach for coffee, you have to poop after 5 + d6 minutes (if the check was random the DM should roll this and surprise the player). You can only hold it in for 1 + d8 minutes starting when you realize.

This determination is semi-permanent, until your stomach adjusts. Every time you drink sage's brew, make a DC20 constitution check to never poop after drinking again!

Half life

All ability score effects halve every 6 hours as the brew wears off.

Remember that in Dungeons and Dragons you always round down, so when a score modifier of 1 is halved it goes to 0.

Jitters

Every time you drink, make a DC5 constitution check to avoid getting the jitters. If you drink more before waiting 3 hours the DC is doubled, this effect stacks. Wait 3 hours for it to go back to 5. The jitters will make you agitated and shaky:

Your charisma score is reduced by 2.

Your dexterity score is reduced by 4.

Stimulant

Your dexterity score is increased by 4. (Unless you have the jitters).

Your intelligence score is increased by 4.

Your constitution score is increased by 2.

Withdrawals

Next time you wake up make a DC5 constitution check or drink more to avoid withdrawals:

Your dexterity score is reduced by 2.

Your intelligence score is reduced by 2.

Every time you drink more the DC for the constitution check increases by 2. Also the resulting score reduction increases by 1 to a maximum of 6.

Sleeplessness

Most creatures cannot sleep for 4 hours after consuming sage's brew. If you have the sage background you can halve this. The first time you drink, make a DC 10 constitution check. If you pass you can divide this period by 2.

This result is permanent as a representation of your brain chemistry.