Mercury does well in the first house especially if Sun also happens to be a co-tenant. Mercury is prince among the planets and first house represents oneself, the physical, mental and character traits of a person. Mercury is an impressionable and young planet according to Vedic Astrology. It takes the characteristics of its co-tenants. Mercury placed alone in the first house behaves like a spoilt prince who seeks patronage without accepting duties.

     First house is ruled by Sun which is a friend of Mercury. In a horoscope the Sun and Mercury move very close to each other. Hence if Sun+Mercury are in the first house than the native does great. If the angle is less then twelve degress then that forms the powerful Budh-Aditya Yoga which gives power, fame, intelligence and splendor. Venus also travels very close to Mercury. Venus with Mercury in the first house gives a chatty, articulate and romantic personality. In a friendly sign Venus+Mercury can be great in business like selling perfurmes, jewelry, music, arts, dancing and other business where feminine grace and splendor are required. Jupiter+Mercury in first house makes native give a great scholar or a very intelligent thug depending on horoscope sign and other associations and aspects. Mercury along with Mars in first house is not bad as long as anger and harsh speech are kept in check. It gives quick thoughts and actions and generally ensure success for the native. Mercury with Saturn in first house may give negative thoughts and speech but it also gives a calculative and disciplined individual who thinks all the pros and cons of a situation before taking desired action. Mercury along with Rahu in first house may give success for few and despair for others depending on horoscope sign. Mercury+Ketu is not bad on a spiritual front but native may need to polish his socializing skills.

     According to B. V. Raman those with Mecury in first house: "Have normal health and fame may spread for them. They receive good education and rise to prominent position due to it. Native maybe interested in occult studies and if Mercury is afflicted can give severe headache to the native".  Mercury in first house generally gives dark or brownish complexion and very talkative personality. Mercury natives have a strong desire to rule but hate associated responsibilities associated with ruling. But their articulate speech and general knowledge is sure to make them successful in most endeavors. They are  less suited to lead unless other planets in horoscope help. 

    Since Mercury is a planet with earth element it may give indigestion, skin problems, swellings and oily skin. It is advisable to consume more water, eat food like lemon juice, apples, green leafy vegetables especially spinach, honey, turmeric, yogurt and other things in appropriate quantity to keep the diseases associated with Mercury away. Also natives should regularly try jal/varun mudra for 5-10 minutes a day. Regular pranayam like Anulom Vilom and Kapal Bhati keep skin problems and indigestion in check. Also the natives should regularly do Surya Namaskar in the morning for wholesome health benefits.

Read our article: Remedies For Mercury to help with weak Mercury.

References:

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