The following is for 14w21b or higher.
Potion food;
1. create a scoreboard objective called "eatfood" and make the stat stat.useItem.bread (It doesn't have to be bread)
2. Make a redstone clock of any kind and hook up some command blocks to it.
3. Put this command into a command block.;
/effect @a[score_eatfood_min=1] <potion> <length> <level>
4. Put this command into another command block;
/scoreboard players remove @a[score_eatfood_min=1] eatfood 1
Custom effect;
This is for a fire potion effect;
1. Add some more command blocks to your clock.
2. Create a dummy objective called custompotion.
3. Write this command in a command block;
/scoreboard players add @a[score_eatfood_min=1] custompotion <potion length>
4. Write this command in another command block;
/execute @a[score_custompotion_min=1] ~ ~ setblock ~ ~ ~ fire
5. Write this command in another command block;
/effect @a[score_custompotion__min=1] 12* 2 0 true

*or whatever the fire resist ID is.