Table 1. Types of interaction of drug combination.

Range of Combination Index	Description	Graded Symbols
<0.1	Very strong synergism	+++++
0.1–0.3	Strong synergism	++++
0.3–0.7	Synergism	+++
0.7–0.85	Moderate synergism	++
0.85–0.90	Slight synergism	+
0.90–1.10	Nearly additive	±
1.10–1.20	Slight antagonism	−
1.20–1.45	Moderate antagonism	−−
1.45–3.3	Antagonism	−−−
3.3–10	Strong antagonism	−−−−
>10	Very strong antagonism	−−−−−