NSAID overdose is not generally associated with significant morbidity. In our experience of 310 consecutive admissions over the last 15 years, no patients have died and only nine have had significant morbidity (4 episodes of renal failure and 5 convulsions) attributable to their NSAID overdose. However, occasionally, large ingestions are reported to cause coma, metabolic acidosis, acute renal failure, bradycardia, hypotension, seizures, or death. Mefenamic acid causes seizures much more commonly than other NSAIDs. There is no specific antidote and treatment is generally supportive. Activated charcoal and generous fluid replacement is usually all that is required.

In therapeutic concentrations, NSAIDs act by competitively inhibiting the enzyme cyclo-oxygenase and preventing the production of prostaglandins from arachidonic acid. This can lead to a large variety of adverse effects in therapeutic use, some of which may occur with a single dose (e.g. gastrointestinal irritation, prolonged bleeding time, asthma, fluid retention in people with intravascular volume depletion, drug interactions). However, cyclo-oxygenase is generally maximally inhibited with therapeutic doses and few of the acute toxic effects are attributable to this action. Thus, toxicity is not necessarily similar between different NSAIDs and it is likely that there is a wide range of toxicity both quantitatively and qualitatively between these drugs. This is evident for mefenamic acid, which causes seizures much more commonly than other NSAIDs. The mechanism for these non-class related effects are obscure.

A number of these drugs will uncouple oxidative phosphorylation at toxic doses, which may partly explain the development of metabolic acidosis and CNS effects.

Most NSAIDs are very rapidly absorbed with peak concentrations in therapeutic use occurring after to 2 hours. There is minimal first-pass metabolism for most of these drugs (except for diclofenac) and so bioavailability is usually greater than 90%.

NSAIDs are generally highly protein bound (>95-99%) and have relatively small volumes of distribution (<0.5 L/kg).

In therapeutic concentrations, NSAIDs act by competitively inhibiting the enzyme cyclo-oxygenase and preventing the production of prostaglandins from arachidonic acid. This can lead to a large variety of adverse effects in therapeutic use, some of which may occur with a single dose (e.g. gastrointestinal irritation, prolonged bleeding time, asthma, fluid retention in people with intravascular volume depletion, drug interactions). However, cyclo-oxygenase is generally maximally inhibited with therapeutic doses and few of the acute toxic effects are attributable to this action. Thus, toxicity is not necessarily similar between different NSAIDs and it is likely that there is a wide range of toxicity both quantitatively and qualitatively between these drugs. This is evident for mefenamic acid, which causes seizures much more commonly than other NSAIDs. The mechanism for these non-class related effects are obscure.

A number of these drugs will uncouple oxidative phosphorylation at toxic doses, which may partly explain the development of metabolic acidosis and CNS effects.

Most NSAIDs are very rapidly absorbed with peak concentrations in therapeutic use occurring after to 2 hours. There is minimal first-pass metabolism for most of these drugs (except for diclofenac) and so bioavailability is usually greater than 90%.

NSAIDs are generally highly protein bound (>95-99%) and have relatively small volumes of distribution (<0.5 L/kg).

There is a wide range of routes of elimination and half-lives for these drugs (see table).

Pharmacokinetic parameters of NSAIDs that may be relevant in overdose

• * Non-linear kinetics
• 1 Active metabolites
• ‡ Extensive enterohepatic recycling
• # Significant first-pass metabolism

Patients are usually asymptomatic but can experience minor gastrointestinal irritation, nausea, vomiting, abdominal pain and diarrhoea.

Macroscopic gastrointestinal bleeding seems to be quite uncommon (c.f. therapeutic use) but has been reported to complicate overdoses of NSAIDs with long half-lives (e.g. phenylbutazone, piroxicam).

Metabolic effects, including respiratory alkalosis, metabolic acidosis, hyponatraemia, hyperkalaemia, hypokalaemia and hypoglycaemia have been reported, in particular with phenylbutazone and oxyphenbutazone. Vomiting may contribute to electrolyte imbalance and dehydration. These metabolic effects may be due to uncoupling of oxidative phosphorylation, thus intracellular and CNS glucose concentrations might be low despite a normal blood glucose. Thus, patients with significant CNS effects should receive an IV glucose bolus to see if they respond.

Mild CNS effects such as nausea, vomiting, tinnitus, confusion and headache are quite commonly reported. Serious complications such as seizures and coma are quite unusual. It should be remembered that CNS effects may be caused by electrolyte disturbance or hypoglycaemia as well as direct toxicity. Seizures occur rarely (< 2%) except in mefenamic acid and phenylbutazone overdoses at any dose but are self limiting.

Acute renal failure has been reported after overdose or single doses of a number of NSAIDs including benoxaprofen, fenoprofen, ibuprofen, mefenamic acid, diclofenac, naproxen, piroxicam, and sulindac.

Renal failure is usually reversible and generally occurs in “at-risk” patients such as those with dehydration, heart failure, liver failure, renal impairment who have relative or absolute intravascular volume depletion. Out of 310 patients with NSAID overdose at our centre, 4 have had acute renal failure, which resolved over several days. Two were young men who had ingested naproxen (7,500 & 25,000 mg) some days previously and only presented when symptoms developed. A unique syndrome of haemorrhagic cystitis has been described with short term use of tiaprofenic acid.

Rises in transaminases have been reported but are not clinically significant. We have had 14/310 patients with NSAID overdose with minor elevations of their ALT, however two had ingested paracetamol and one a very large amount of ethanol.

The bleeding time is prolonged due to a reversible inhibition of thromboxane production leading to reduced platelet aggregation. NSAIDs differ in duration of effect from less than 1 day for those with short half lives to 2 weeks or more for those with long half lives. (See Pharmacokinetic values)

In large or clinically severe overdoses the following should be monitored:

• full blood count
• electrolytes & glucose
• renal function
• Liver function tests
• arterial blood gases
• coagulation studies
• ECG
• paracetamol level

Abnormalities occur rarely, but appear to be most frequent in mefenamic acid and phenylbutazone overdoses.

NSAID plasma concentrations are not widely available and have not been shown to be useful for the evaluation and management of overdose.

It is likely that many of these drugs have their own spectrum of toxicity (as toxicity is mostly not due to the effect on cyclo-oxygenase that defines this class); however, little experience with many of these drugs has been reported. The high risk of seizures and other CNS effects with mefenamic acid and the generally more serious nature of phenylbutazone overdose are pertinent examples.

IV access and fluids should be given to patients who are dehydrated or unable to ingest adequate fluids. Avoiding any intravascular fluid depletion is likely to help prevent the development of renal failure. Abnormalities of electrolytes should be corrected.

Not clinically useful and contraindicated following mefenamic acid overdose because of the risk of imminent seizures.

There are no specific antidotes for the most serious manifestations. The gastrointestinal effects could presumably be antagonised by a proton pump inhibitor.

Seizures should be treated with IV glucose (as there may be an uncoupling of oxidative phosphorylation, with resultant intracellular hypoglycemia) and then benzodiazepines (e.g. diazepam 5 to 10 mg IV).

Most of these drugs have short half-lives and there is unlikely to be significant benefit from repeated doses of activated charcoal except perhaps for phenylbutazone and sulindac. Other methods of elimination enhancement, such as haemodialysis and haemoperfusion are not warranted.

Adults following deliberate self poisoning who are asymptomatic with normal vital signs at 6 hours post ingestion are fit for discharge.

Children do not develop significant symptoms until ingestion dose exceeds 400 mg/kg. Minor unintentional ingestions of up to 3 tablets do not require referral to hospital.

Death is extremely unusual but has been reported for fenoprofen, ibuprofen, and phenylbutazone.

Serious long term sequelae have not been reported and no follow up would normally be required after resolution of the clinical signs. The incidence of delayed renal injury is unknown.

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