I followed the potion recipes I found for healing a weakened villager, and all they do is blow up and make the person mixing it weaker.

I had my master scientist, whose like is alchemy, mix cactus-cactus-lotus, and it blew up and took away half her health. I had my other master scientist mix the other recipe I found for fixing weakened villagers, cactus-lotus-lotus, and it blew up on her too, and took away half HER health!

I have level 2 in all my techs except leadership, and level one in my nature faction. Am I missing something?

Until you get to level 3 of Alchemy Tech, there is a risk of explosion. It has nothing to do with the combination of herbs you're using. The best thing to do is try again using a different (healthy) villager.

When you don't have the level 3 of alchemy, the potions may blow up and it's random!

So I've just had bad luck then. Okay.

It doesn't appear I can use any doctors/healers to cure weakness, so I assuming it can only be repaired with the potion?

With patience! You may give them a task that's not hard as Farming or Healing and make them eating a lot they will go better in a few time.

Cool. Thanks.

You're welcome!