Execute sua declaração no seguinte formulário:
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'myproc') AND type in (N'P', N'PC'))
DROP PROCEDURE [dbo].[myproc]
GO
create PROCEDURE [dbo].[myproc]
AS
BEGIN
select * from mytable
END
GO
Observe o
GO
separador de lote após DROP PROCEDURE