The drink is made by Benedictine monks in Devon but is bottled and distributed by J Chandler & Company (Buckfast).
Annual accounts for that business show turnover was down more than 4% from £39.1 million to £37.4m in the 12 months to March 31, 2013, while pre-tax profits were relatively flat at £2.74m.
Writing in the accounts, directors suggested the decline in turnover may be because of alcohol duty increases from March 2012 which forced them into putting the price of Buckfast up.
The executives at the Andover, Hampshire, company said they hoped sales would stabilise in the current year.
A bottle of the tonic wine costs in the region of £7.
There were £103,900 of charitable donations in the year, which included £50,000 given to Cumbernauld-based youth football club Carbrain.
J Chandler remains in a strong financial position, with its net funds increasing from £16m to more than £17.26m.
Directors' remuneration increased from £2.14m to in excess of £2.5m, with the highest paid receiving £335,000, which was down from £348,000 in the previous year.