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Many drugs interact with neuromuscular blocking drugs and often enhance the induced block; this is of clinical importance for volatile anaesthetics, antimicrobials, magnesium and some more specific drugs. Difficulty in reversing the block occurs with calcium-channel blockers and polymyxin. Phenytoin, carbamazepine and other anticonvulsants may cause resistance to neuromuscular blocking drugs. Moreover, clinically important interactions are found between individual neuromuscular blockers. Giving succinylcholine after a non-depolarizing neuromuscular blocking drug prolongs the onset of succinylcholine; when non-depolarizing drugs are administered after succinylcholine their effects are prolonged. The succinylcholine block is prolonged when the drug is administered during recovery from pancuronium or following neostigmine reversal. Drugs or diseases that decrease the activity of plasma cholinesterase may prolong a succinylcholine-induced block. Finally, liver dysfunction, renal failure, disturbances of acid-base balance, change in temperature and neurological diseases all have an effect on the profile of the neuromuscular blocking drugs; the response to an induced block may be altered in patients under intensive care and those with cancer. Although knowledge of the most important theoretical interactions of neuromuscular blocking drugs is favourable, the anaesthetist should be aware that pharmacological interactions can lead to an unpredictable induced neuromuscular block in many cases in daily clinical practice. Therefore anaesthetists should become familiar with the use of neuromuscular transmission monitoring in order to manage the block correctly.

Aging alters both the pharmacokinetic and the pharmacodynamic aspects of anesthetic requirement. Studies of the relationship between drug concentration and effect in older adults clearly demonstrate a decline in median effective dose requirement for agents that act within the central nervous system, but there appears to be little change in the dose required for peripheral effects such as neuromuscular blockade. Most drugs also undergo somewhat slower biotransformation and demonstrate prolonged clinical effects if they require hepatic or renal degradation, although many newer agents such as remifentanil and cisatracurium have organ-independent pathways that are not affected by age. In some cases, however, the appearance of increased sensitivity to a given dose of anesthetic or opiate may actually reflect higher-than expected plasma concentrations of drug following a rapid intravenous injection. Therefore, it is impossible to completely separate the interactions between pharmacodynamic and pharmacokinetic factors associated with aging. The use of pharmacological sympathectomy with intrathecal agents and with sympatholytic adrenergic agonists may further improve outcome in a patient population at high risk because of reduced functional reserve, increased incidence of polypharmacy, and the consequences of age-related disease.

Noncompartmental pharmacokinetic methods using the statistical moment theory can be very helpful in comparing the mean residence time of drugs in the body. These methods are based on the estimation of the area under the time versus plasma concentration curve after a single intravenous drug bolus, and can also be used to estimate clearance, effective half life and the apparent volume of distribution of the drug. In the group of nondepolarising neuromuscular blocking drugs, atracurium offers the advantage of having the smallest mean residence time.