Assalam O Alaikum,
Cleaning the stomach by VOMITING was a patent treatment in the past.
A popular method among Hakeems was :
Luke warm water having a lot of salt is given to the patient, he drinks a glass, then a glass and a glass again. Hot/salty water brings out the vomit by Allah's Will.
Salty water should be continued after vomit starts; till the last vomit comes out similar to the color & turbidity of the water drunk.
There had been other methods which are unknown to me, some say to insert the finger in your mouth till it touches the throat & vomit comes out, but this seems painful.
Take reetha and boil it in water for few minutes. Gargle it and you will vomit in no time.
Even you can use zeera water luke warm.
hope this help.