potion of weakness without opportunity to be in survival mode snapshot 15w51b

after going to the nether by rods blaze and do my laboratory have managed to overkill was making a potion of weakness making a strength and time to put an eye spider fermented the Alchemy machine did not work and not to me try the potion but with others such as speed potion and if I run gave me the negative and other potions, and so I tried the potion except weakness.
I'm on a windows 8 4 gb ram operates hgz 2.16 64 bits and is minecraft pc edition