The most common adverse effects of the MAOIs leading to discontinuation of these drugs are orthostatic hypotension and weight gain. In addition, the irreversible nonselective MAOIs are associated with the highest rates of sexual effects of all the antidepressants. Anorgasmia is fairly common with therapeutic doses of some MAOIs. The amphetamine-like properties of some MAOIs contributes to activation, insomnia, and restlessness in some patients. Phenelzine tends to be more sedating than either selegiline or tranylcypromine. Confusion is also sometimes associated with higher doses of MAOIs. Because they block metabolism of tyramine and similar ingested amines, MAOIs may cause dangerous interactions with certain foods and with serotonergic drugs (see Interactions). Finally, MAOIs have been associated with a sudden discontinuation syndrome manifested in a delirium-like presentation with psychosis, excitement, and confusion.
Suicide attempts are a common and unfortunate consequence of major depression. The lifetime risk of completing suicide in patients previously hospitalized with MDD may be as high as 15%. Overdose is the most common method used in suicide attempts, and antidepressants, especially the TCAs, are frequently involved. Overdose can induce lethal arrhythmias, including ventricular tachycardia and fibrillation. In addition, blood pressure changes and anticholinergic effects including altered mental status and seizures are sometimes seen in TCA overdoses. A 1500 mg dose of imipramine or amitriptyline (less than 7 days’ supply at antidepressant doses) is enough to be lethal in many patients. Toddlers taking 100 mg will likely show evidence of toxicity. Treatment typically involves cardiac monitoring, airway support, and gastric lavage. Sodium bicarbonate is often administered to displace the TCA from cardiac sodium channels.
An overdose with an MAOI can produce a variety of effects including autonomic instability, hyperadrenergic symptoms, psychotic symptoms, confusion, delirium, fever, and seizures. Management of MAOI overdoses usually involves cardiac monitoring, vital signs support, and lavage.
Compared with TCAs and MAOIs, the other antidepressants are generally much safer in overdose. Fatalities with SSRI overdose alone are extremely uncommon. Similarly, SNRIs tend to be much safer in overdose than the TCAs. However, venlafaxine has been associated with some cardiac toxicity in overdose and appears to be less safe than SSRIs. Bupropion is associated with seizures in overdose, and mirtazapine may be associated with sedation, disorientation, and tachycardia. With the newer agents, fatal overdoses often involve the combination of the antidepressant with other drugs, including alcohol. Management of overdose with the newer antidepressants usually involves emptying of gastric contents and vital sign support as the initial intervention.
Antidepressants are commonly prescribed with other psychotropic and nonpsychotropic agents. There is potential for drug interactions with all antidepressants, but the most serious of these involve the MAOIs and, to a lesser extent, the TCAs.
A. Selective Serotonin Reuptake Inhibitors
The most common interactions with SSRIs are pharmacokinetic interactions. For example, paroxetine and fluoxetine are potent CYP2D6 inhibitors (Table 30–4). Thus, administration with 2D6 substrates such as TCAs can lead to dramatic and sometimes unpredictable elevations in the tricyclic drug concentration. The result may be toxicity from the TCA. Similarly, fluvoxamine, a CYP3A4 inhibitor, may elevate the levels of concurrently administered substrates for this enzyme such as diltiazem and induce bradycardia or hypotension. Other SSRIs, such as citalopram and escitalopram, are relatively free of pharmacokinetic interactions.