Patients on beta blockers, especially propranolol, may be far less responsive to adrenaline, thus may be difficult to resuscitate if they have an anaphylactic reaction to antivenom. While never documented with antivenom, this adverse effect of beta blockers has been reported with some other causes of anaphylaxis, with potentially lethal consequences. This should be considered before deciding to commence antivenom therapy. If in doubt, seek expert advice.
Though less clear, a similar situation may apply to ACE inhibitors.