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is more related to the mental status of a person and it is not a disease in most cases.The problem arises because of mental stress;work pressure or stress;family issues;mind occupied with multiple issues which distract the attention from sex thus causing feeling of weakness.The better option is to relax your mind and to increase timing you can do Kegel's exercise also try the stop/start method (for details consult a sexologist
or your family doctor ;even yoga
helps to control mind.As for herbal medicines many are available over the counter but none has guaranteed or proved result that will last life long and they function more like other aphrodisiac medicines.
Its your pick ;I hope i have answered your query.