Nothing's 100%, sorry. See this news article about someone who survived falling out of an airplane at 33,000 feet (yes, a 10 kilometer fall).
https://www.bbc.com/news/world-europe-38427411
That being said, India (like many countries with a large percentage of the population working in agriculture) has a large portion of suicides occuring by poisoning from the industrial strength pesticides used by farmers... it's reported to be very painful, though. And for some reason, there's also a relatively high proportion of suicide by self-immolation in India... which must make death by pesticide pleasant in comparison.
Otherwise, suicide methods in India are more or less the same as anywhere else. Hanging, jumping, drowning, cutting. I suspect that if there were a readily available, relatively painless pill that was 100% fatal, there wouldn't be millions of suicide survivors every year.