The aim of our study is to define the median effective analgesic doses (ED50) of paracetamol, morphine, and their combination and determination the nature of their interaction administered IV for postoperative pain after moderate painful surgery using up-and-down and isobolographic methods.

Ninety patients scheduled to undergo moderately painful surgery were included in one of three groups. Determination of median effective (ED50) doses was performed by the Dixon and Mood up-and-down method. Initial doses were 1.5g and 5mg, with dose adjustment intervals of 0.5g and 1 mg, in the paracetamol and morphine groups, respectively. The initial doses of paracetamol and morphine were 1.5g and 3mg, respectively in the paracetamol-morphine combination group with dose adjustment intervals of 0.25g for paracetamol and 0.5mg for morphine. Analgesic efficacy was defined as a decrease to less than 3 on a 0-10 numeric pain scale, 45 min after the beginning of drug administration. Isobolographic analysis was used to define the nature of their interaction.

Inclusion Criteria:

1. ASA physical status І or ІІ
2. Moderately painful surgery patients
3. postoperative pain more than 3 on a numerical pain score (NPS) -

Exclusion Criteria:

1. contraindication to the use of paracetamol and morphine
2. Pregnancy
3. age younger than 18 yr
4. patient weight less 65kg
5. intra-operative regional anesthesia,
6. intra-operative administration of analgesics other than fentanyl,
7. postoperative pain less than 3 on a numerical pain score (NPS)