Shares in the new company Alliance Boots began trading at 8am this morning.
Cost synergies of at least£100 million a year by the fourth year are expected from the tie-up.
The combined company has more than 3,000 retail outlets, of which about 2,700 have pharmacies attached, and operates 400 pharmacies outside the UK.
A range of popular Boots products will be available in 500 Alliance pharmacies from today.
Alliance Boots chief executive Richard Baker said: 'The union of these two great businesses establishes a new market leader in UK pharmacy and provides exciting opportunities for international growth.'
The group issued trading updates for the last months before the merger became effective.
For the three months to June 30, like-for-like sales at Boots the Chemists were up 3 per cent. Boots' launch of its Health Club in April exceeded expectations and 1 million people have signed to the initiative.
Revenue from Alliance UniChem's retail division grew by an estimated 4 per cent for the first half of the financial year. Its wholesale business grew by about 3.5 per cent.