You cannot use Senna for a long time. It contains certain substances called anthraquinones which stimulate the colon to contract and help expel the stool. Ideally, this should not be used for more than 1 week consecutively. An important side effect of Senna with long term use is a condition known as lazy bowel syndrome, which is a condition in which you will be unable to pass stools without a laxative.
It will be better for you to use another herbal product called Psyllium husk, available as Metamucil. This is biologically inert and can be used safely for years. It increases the bulk of the stool which then increases the pressure inside the rectum, stimulating the passage of stool.
You will need to drink at least 2 glasses of water with eash tsf of psyllium to get the full benefit.
I hope this helps.
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