Eight house is the house of death, transformation and hidden secrets. Mercury though smart and analytical is way too talkative and not the right planet to handle the portfolio's associated with eight house. With planets like Sun or Jupter it may provide good results.

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     According to B. V. Raman: "Mercury in the eight house blesses the native with fine speech and courteous demeanor along with good inheritance. Native is long lived, a scholar and earns well. There is likely chances of bad health and weak constitution." Eight house is the house of hidden secrets. Mercury is smart and posseses extra-ordinary analytical skills. The native maybe able to learn and decipher a lot of mysteries and secrets. That is why a career as a detective, scientists, forensic expert, astrologer or as an archeologists are suited. Mercury in eight house can make an excellent engineer and a software developer if Mercury is the lord of tenth house or sixth house or associated with those lords.

     Ideally Mercury is smart but there is a tendency to show off and blurt out often. Matters of eight house are best left as a secret but Mercury often fails to realize this. Often there could be troubles if secret informaton or hidden knowledge is leaked. Natives with Mercury in eight house should try their level best not to reveal too much to others especially those who can misuse their knowledge. Mercury with Sun in eight house is desirable but in a bad sign there could be digestion problems as well as piles. Mercury with Moon in eight house is not good and may give nervous disorders and weak mind. Native should regularly eat Vigna Radiata(Mung Beans) to avoid mental troubles. Mercury with Mars in eight house is very bad for health and gives digestion problems similar to Mercury with Sun but it could also give troubles with in-laws. There is a chance of divorce and spouse is likely to have a direct and rude speech. Mercury with Jupiter in eight house gives digestive troubles but this association is not known to cause too much harm. Mercury with Venus in eight house gives articulate, aristocrat and fashionable spouse and in-laws. Multiple spouse and relationships are indicated if there is Mercury+Venus along with influence of Rahu or Mars. There are chances of getting venereal disease. Mercury with Ketu can give trouble with in-laws and increases chances of divorce. Mercury with Rahu is not bad and may give good results. Mercury with Saturn is great until old age. In old age Mercury's association with Saturn in eight house has been known to cause troubles.

     Mercury in eight house has its benefits as well as drawbacks. There is a weak constitution and regular exercise as well as Kapal Bhati Pranayam is essential. Yogas and exercise which aid digestion should be done on regular basis. Foods such as yogurt, honey, mung beans, sprouted beans. spinach, apple and other foods should be consumed regularly to help keep digestive problems at bay. Also one must not do heavy workouts if it causes digestive problems. Natives should try to avoid talks associated with eight house like inheritance, occult knowledge, taxation to avoid troubles later. Prayers to Lord Vishnua are best for natives with Mercury in eight house. Additionally Gayatri Mantra and prayers to Sun are also good for natives to help with health troubles. 

References:
Raman, Bangalore Venkata. How to Judge a Horoscope. Delhi: Motilal Banarsidass, 2014. Print.