What happens if we give phenytoin and neostigmine

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I now have enough information from the textbooks. Here is the complete answer:

Phenytoin + Neostigmine: Drug Interaction

This is a context-dependent interaction that plays out primarily at the neuromuscular junction (NMJ), and the outcome depends on which neuromuscular blocking agent (if any) is also present.

The Core Interaction

Drug	Mechanism
Phenytoin	Anticonvulsant; membrane stabilizer; sodium channel blocker; also induces CYP3A4
Neostigmine	Acetylcholinesterase (AChE) inhibitor; increases ACh at the NMJ

These two drugs have opposing effects at the NMJ and tend to antagonize each other in that setting.

Scenario 1: Reversal of Nondepolarizing Blockade (e.g., vecuronium, rocuronium)

This is the most clinically relevant context.

Neostigmine is used to reverse nondepolarizing neuromuscular blockade by increasing ACh at the NMJ.
Phenytoin reduces/antagonizes nondepolarizing blockade on its own (acts as a resistance factor).
Together, both agents work toward the same end - reversing or resisting nondepolarizing blockade - but through different mechanisms.

From Morgan & Mikhail's Clinical Anesthesiology (7e), Table 11-3 confirms:

Anticonvulsants (phenytoin, carbamazepine, valproate): Unknown effect on depolarizing block; cause resistance (-) to nondepolarizing blockade
Cholinesterase inhibitors (neostigmine, pyridostigmine): Potentiate (+) depolarizing block; reverse (-) nondepolarizing blockade

Scenario 2: Context of Neuromuscular Junction Disorders (e.g., Myasthenia Gravis)

This is where the combination can be dangerous.

In patients with MG or other NMJ transmission disorders, neostigmine is a therapeutic agent (increases ACh to compensate for reduced receptor density).
Phenytoin compromises the safety factor for neuromuscular transmission and is listed in Goldman-Cecil Medicine as a drug that worsens or induces neuromuscular transmission disorders.
Giving phenytoin to a patient already on neostigmine for MG can counteract the therapeutic benefit of neostigmine and worsen weakness.

Scenario 3: Depolarizing Block (Succinylcholine)

From the Harriet Lane Handbook:

Neostigmine (as an anticholinesterase) antagonizes the effects of nondepolarizing blockers like vecuronium.
Phenytoin also antagonizes the effect of nondepolarizing blockers.
For depolarizing blockade (succinylcholine), neostigmine potentiates it (by blocking the pseudocholinesterase that breaks down succinylcholine, and by increasing ACh which intensifies the depolarization). Phenytoin's effect on depolarizing block is listed as unknown (?).

Mechanism Summary

Phenytoin stabilizes neuronal membranes, reduces frequency potentiation at the NMJ synapse, and - via CYP3A4 induction - accelerates metabolism of some NMJ blockers. This reduces the safety factor for neuromuscular transmission.
Neostigmine increases ACh by blocking AChE, enhancing neuromuscular transmission.
Their combined effect = neostigmine's ability to build up ACh is partially offset by phenytoin's membrane-stabilizing and synaptic-suppressing action.

Key Clinical Points

In patients on chronic phenytoin therapy, you may need higher doses of nondepolarizing muscle relaxants (phenytoin causes resistance), and reversal with neostigmine is still effective but the underlying pharmacodynamics are shifted.
In myasthenia gravis, phenytoin is listed as a drug that can worsen NMJ transmission - this directly opposes neostigmine's therapeutic goal. Avoid phenytoin in MG patients if possible.
If both are given without a neuromuscular blocking agent in the picture, the direct pharmacological conflict is at the NMJ: neostigmine tries to enhance cholinergic transmission; phenytoin suppresses it.

Sources:

Morgan and Mikhail's Clinical Anesthesiology, 7e, Table 11-3 (Drug Interactions, NMJ blockers)
Harriet Lane Handbook, 23rd ed., p. 10276 (Vecuronium drug interactions)
Goldman-Cecil Medicine, Table 390-2 (Drugs that worsen neuromuscular transmission disorders)
Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e, p. 283 (Phenytoin and CYP3A4 induction at NMJ)

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