Fukusū Pill

The Fukusū pill was a prototype pill created by Vegapunk that would potentially make it so someone may eat more than one devil fruit and gain more devil fruit powers. Unfortunately it came with a lot of conditions and side-effects and was discarded by Vegapunk, not to be seen again until the Sunglass Pirates would eventually consume these pills and all become users of three devil fruits



Conditions
There were a lot of things that go along with this however. The maximum amount of fruits one may eat with this pill is 3, one Logia, one Paramecia and one Zoan. Also, this must be done in a time of under 5 minutes, because by then the pill wears off and to take another dose of it would be fatal. Another thing is that the person eating the pill cannot already have a devil fruit power, otherwise the pill will not work and the devil fruit power already inside the person will be weakened. Another negative is that once they take more than one devil fruit, they will be weakened by even slight amounts of water, such as rain or dipping their claves in the water. Along with that, the physical side-effects include insanity and blindness in some cases, slower thinking in most cases, and a shortened lifespan in all cases.

However the ability to have 3 devil fruit powers is tremendously useful, especially when combining a Zoan with wings and a Logia. The overall amount of moves you can execute with 3 different types of devil fruits is tremendous.