Capsule

Megavitamins come in six different color combinations of red, blue, or yellow (Red+red, blue+blue, yellow+yellow, red+blue, yellow+blue, red+yellow, or in the Game Boy version Black+black, grey+grey, white+white, black+grey black+white and white+grey). Dr. Mario uses the megavitamins to destroy Viruses, in the game Dr. Mario. In Japanese, Megavitamins are simply called カプセル(Kapuseru); Capsules.

In Super Smash Bros. Melee, Dr. Mario's neutral B attack makes him shoot Megavitamins. They act the same as Mario's Fireball move, except that the Megavitamins are more powerful. In fact, they are one of the strongest fast-moving projectile in the game. They also bounce in a different path, and make a different noise when it hits. There's also a trophy of Dr. Mario shooting Megavitamins and one of the vitamins themselves. Kirby is also able to use the Megavitamins if he copies Dr. Mario's powers (via attack), although the SFX that plays when the Megavitamin hits an opponent is different.

In Dr. Mario 64, Megavitamins also seem to have a transforming property; when Dr. Mario or Wario ate them, they would transform into Metal Mario and Vampire Wario, respectively. This may be an aftermath of the Megavitamins attempting to destroy them like it destroys Viruses. In Super Smash Bros. Brawl they appear as Stickers.

Trivia
In dr luigi the red megavitamin becomes purple/pink
 * There is a planet in Good Egg Galaxy in the game Super Mario Galaxy that resembles a Megavitamin.
 * One of the things thrown at Vanessa in WarioWare: Touched! is a red+blue Megavitamin.