Mercury in the 6th house (Ari / Shatru, the house of health & service) brings the significations of Mercury, namely intellect & analysis, communication & speech, business & trade, into the area of health, service, competition and the obstacles you overcome. The 6th house governs enemies & competition, disease & health, debts, daily work & service.
Results of Mercury in the 6th House
Mercury is a neutral planet, so here it tends to bring poor communication, indecision, nervousness and trouble in studies or trade focused on health, service, competition and the obstacles you overcome. As the 6th is a dusthana (difficult) house, results may come through struggle before growth.The exact result depends on which sign Mercury occupies here and the aspects it receives.
When It Activates
These results typically surface during the Vimshottari mahadasha or antardasha of Mercury, and when transiting planets trigger the 6th house. A well-placed Mercury here supports the power to defeat enemies, good health with effort and success in service; when afflicted it can indicate chronic illness, mounting debts and strong opposition from rivals.