Two Analgesics, One Molecule
Mrs F is 74, weighs 42 kg, and is day two after a laparoscopic anterior resection. She is prescribed regular IV paracetamol 1 g four times daily, co-codamol 30/500 two tablets four times daily as required for breakthrough pain, and ibuprofen 400 mg three times daily. Her eGFR is 55 mL/min/1.73m² and she has a documented history of duodenal ulcer. At 14:00 she rates her pain 6/10 and asks for 'something stronger'. Her abdomen is soft, she is apyrexial, and she has opened her bowels.
1. Calculate the maximum daily paracetamol dose if she receives both the regular IV paracetamol and the maximum PRN co-codamol. Express this in mg/day and mg/kg/day, and compare it with the licensed limit for an adult under 50 kg.
Read the worked answer
Step 1 - regular paracetamol 1 g × 4 doses = 4000 mg/24 h
Step 2 - paracetamol hidden in the PRN Co-codamol 30/500 contains 500 mg paracetamol per tablet. 2 tablets = 1000 mg paracetamol per dose. 1000 mg × 4 doses = 4000 mg/24 h
Step 3 - total 4000 + 4000 = 8000 mg/24 h 8000 ÷ 42 kg = 190.5 mg/kg/24 h
Step 4 - the licensed ceiling for adults under 50 kg
- 15 mg/kg per dose = 15 × 42 = 630 mg per dose
- Maximum 60 mg/kg/24 h = 60 × 42 = 2520 mg/24 h
Conclusion The maximum possible intake, 8000 mg/day, is more than three times the licensed ceiling of 2520 mg/day. Even the regular prescription alone (4000 mg = 95 mg/kg/day) exceeds it. This is a paracetamol overdose waiting to happen, and it is not obvious on the chart because the two prescriptions look like different medicines.
2. Which of her prescribed medicines is most unsafe, and what action do you take before administering anything?
Read the worked answer
Two distinct problems
Paracetamol duplication and weight-inappropriate dosing. She has crossed the threshold for hepatotoxicity risk if every dose is given. The 1 g dose is itself inappropriate at 42 kg.
Ibuprofen is the most clearly contraindicated single medicine.
- Documented duodenal ulcer - NSAIDs are contraindicated: risk of GI bleeding and perforation.
- eGFR 55 with recent surgery and a 74-year-old - NSAIDs risk acute kidney injury and fluid retention. Also flag that co-codamol at 2 tablets four times daily gives 240 mg codeine/day, which causes constipation, sedation and falls in an older adult.
Actions before administering
- Withhold the PRN co-codamol and the ibuprofen; continue to give prescribed regular analgesia only if it is safe - but the 1 g paracetamol also needs review.
- Contact the prescriber, present the numbers plainly: total potential paracetamol 8 g/day, 190 mg/kg/day against a ceiling of 2.5 g for her weight; ibuprofen contraindicated by ulcer history and renal function.
- Ask for a weight-adjusted prescription: paracetamol 630 mg IV four times daily (max 2520 mg/day), and removal or replacement of co-codamol so paracetamol is not duplicated - with a laxative if an opioid is continued.
- Never adjust the dose yourself; document the query, the response and the time.
- If any doses were already given, escalate and consider liver function monitoring; complete an incident report.
3. What analgesic and non-pharmacological alternatives would you propose to the prescriber?
Read the worked answer
First, check the pain is what it seems 6/10 on day two after laparoscopic surgery - assess at rest and on movement, and screen for something new: wound haematoma or infection, anastomotic leak (abdominal pain with a soft abdomen is reassuring but not conclusive), urinary retention, constipation, or anxiety amplifying pain.
Optimise what is safe
- Regular, weight-adjusted paracetamol (630 mg four times daily) given round the clock rather than waiting for pain to build.
- Avoid NSAIDs entirely given the ulcer and eGFR 55.
- If an opioid is needed, suggest a low-dose immediate-release opioid (e.g. oral morphine 2.5–5 mg) titrated to effect, with a laxative and antiemetic, and monitoring of sedation, respiratory rate and bowel function. This avoids the hidden paracetamol in co-codamol.
- Review hydration and renal function, and avoid other nephrotoxins.
Non-pharmacological measures you can start now
- Wound support with a pillow when moving or coughing.
- Positioning, early mobilisation and breathing exercises.
- Heat, distraction, relaxation and good sleep hygiene at night.
- Ask the surgical team about local anaesthetic wound infiltration or a regional block, which can substantially reduce opioid requirements.
Communication Tell her honestly what you are doing and why - that you are sorting out a safe combination rather than refusing her pain relief, and give her a realistic time frame for the review.