What counts as damage for the no damage achievements?

Question

In the Binding of Isaac, there are various things that decrease total health in some way. Which of those things count as damage for the "Complete the X without taking damage achievements" (Basement Boy, Spelunker Boy, Dark Boy, and Mamas Boy)?

For example, if I take damage going into and out of a curse room, can I still get the achievements? Or if I use a blood donation machine?

Answer 1

From the wiki:

What counts as "taking damage?"

• Getting hit by an enemy, enemy projectile, bomb (pretty much what you would expect)
• Purchasing an item from the Devil Room.
• Using a "Health Up" pill.

What doesn't count as "taking damage?"

• Using the Blood Donation Machine or IV bag.
• The "Bad Trip" Pill effect.
• The Kamikaze! item.

Sadly, only one of your two examples is explicitly covered above, though I'd imagine the curse room falls under the very first bullet of "taking damage", along with the spike room (I'd try this myself but actually managing to take no damage is incredibly difficult, more so when you have to keep track of meta-game things).

Also, the wiki doesn't say anything about newer "sacrifice your health items", such as the Razor or Blood Rights. I've never used these items myself and qualified for one of the achievements, so I cannot attest to whether or not they count as damage (if I had to guess, I'd say no as per Kamikaze!).

Finally, the bit about the "Health Up" pill seems strange, but I've never paid specific attention in order to verify this (and setting this up is quite difficult).

Answer 2

According to the wiki:

What counts as "taking damage?

• Getting hit by an enemy, enemy projectile, bomb (pretty much what you would expect)
• Purchasing an item from the Devil Room.
• Using a "Health Up" pill.

What doesn't count as "taking damage?

• Using the Blood Donation Machine or IV bag.
• The "Bad Trip" Pill effect.
• The Kamikaze! item.

Other information

• Taking damage in an area will not disable all the boy achievements, only the one applicable to the current area. (you can take damage in the Basement and still unlock Spelunker in the Caves)

Answer 3

According to the mechanics guide, the only sources of damage that count against no damage achievements are damage from enemies and deals with Satan. Importantly,

You do NOT lose credit for the "no hits taken" achievement when

• You use a blood donation machine
• You help a devil beggar
• You take a Bad Trip! pill
• You use the Razor Blade item
• You use the Blood Rights item
• You use the IV Bag item