Number One

Number One is a passive item.

Effects[]

Brimstone/ Tainted Azazel: The laser turns yellow and charges at a much faster rate with no decrease to the range.
Epic Fetus: Increases fire-rate with no decrease in range. Explosive clouds are now yellow.
Incubus: The Incubus fires at a much faster rate with no decrease to the range.
Ipecac: Increase fire rate moderately and decreases range only slightly.
The Ludovico Technique: Turns tear yellow and allows it to be moved much faster.
Pop!: Eyeball tears are yellow, can be fired much quicker, and does not affect their airtime.
Technology: The laser turns yellow and Technology's firing rate is greatly increased with no decrease to the range.
Tech X: Laser turns yellow, charge time is greatly decreased, and no decrease to the range.
Tiny Planet: Tears travel roughly the same distance but can be fired much quicker.

Azazel: Lowers range, reduces charge time.
Compound Fracture/ The Parasite: Tears often disappear before being able to split. Split tears also only travel a very short distance.
Cricket's Body: Tears disappear very quickly after being fired and split tears barely travel at all.
Dr. Fetus: Not recommended. Bombs do not travel far, making self-damage very likely.
Haemolacria: Does not affect the main tear; however, burst tears travel significantly less far.
Kidney Stone: Number One's range penalty overrides Kidney Stone's. The stone is not affected by short range, otherwise the combo behaves as expected.
A Lump of Coal: Tears still increase in size and damage as they travel; however, the short-ranged distance limits tear size and damage.
Marked: Tears become orange. The firing rate is increased, Marked's effect is unaffected but the range is reduced.
Mom's Knife: The knife turns yellow and the maximum range is significantly reduced.
Monstro's Lung/ Lead Pencil: Not recommended. Tear barrage disappears very close to Isaac.
- Range is more forgiving.
Proptosis: Not recommended. Tears disappear after traveling a very short distance.
- This is now fixed and works as expected.
Rubber Cement: Tears often disappear before being able to bounce off walls.
Toothpicks: Tears become orange.

"Number One" is a euphemism for urination.
In the original Binding of Isaac, Number One had the effect of turning all future Range Down pills into Range Up pills. This was presumably designed to reduce the risk of players being stuck in situations where their range would become too low to clear certain rooms. However, in Rebirth, Number One no longer has this effect, and will not turn Range Down pills into Range Up pills.
This item is used in High Brow (challenge #2).
The firing animation of Number One is a little different from what it looks like in the original Binding of Isaac.