UPDATE Tovar
Tovar
LEFT JOIN (
SELECT * FROM Tovar WHERE Tovar.dozirovka IS NOT NULL
) tmp
ON Tovar.md5Drug = tmp.md5Drug
AND Tovar.idCountry = tmp.idCountry
AND Tovar.idForma = tmp.idForma
AND Tovar.idManufacturer = tmp.idManufacturer
AND Tovar.dozirovka IS NULL 
SET Tovar.dozirovka = tmp.dozirovka

UPDATE Tovar
Tovar INNER JOIN
(SELECT Tovar.* FROM (SELECT * FROM Tovar T
WHERE T.dozirovka IS NULL
AND EXISTS (SELECT 1 FROM Tovar T2 WHERE T2.md5Drug = T.md5Drug AND T2.dozirovka IS NOT NULL
AND T2.idCountry = T.idCountry
AND T2.idForma = T.idForma
AND T2.idManufacturer = T.idManufacturer
AND T2.id<>T.id
)) as tmp
INNER JOIN Tovar
ON Tovar.md5Drug = tmp.md5Drug
AND Tovar.idCountry = tmp.idCountry
AND Tovar.idForma = tmp.idForma
AND Tovar.idManufacturer = tmp.idManufacturer
WHERE Tovar.dozirovka IS NOT NULL
GROUP BY Tovar.md5Drug,Tovar.dozirovka
ORDER BY NULL) tmpS
ON  Tovar.md5Drug = tmpS.md5Drug
AND Tovar.idCountry = tmpS.idCountry
AND Tovar.idForma = tmpS.idForma
AND Tovar.idManufacturer = tmpS.idManufacturer
AND Tovar.dozirovka IS NULL
SET Tovar.dozirovka = tmpS.dozirovka