Negative potion affects are not applied when a potion is drunk

Negative potion effects (wither, poison, nausea, etc) do not apply to a character when the potion is drunk, but positive effects do (strength, speed, etc)

What I expected to happen was...:
When I drank a slowness I, strength I, resistance I, I would get all the effects

What actually happened was...:
I only got the strength and resistance, not the slowness

Steps to Reproduce:
1. Obtain a program such as mcedit or NBTedit
2. Modify the potion to have custom effects (in this case a negative one)
3. Observe that the effect is displayed on the tool tip
4. Drink the potion
5. No effect