Birth control pills: Some antifungals, antibiotics, anticonvulsants and barbiturates can reduce the effectiveness of oral contraceptives, resulting in as much as a 25 fold increased risk of pregnancy. The Pill has also been found to increase the blood levels of many antidepressants, leading to potential overdose.
Anti ulcer drugs: Cimetidine, marketed as Tagamet, can be dangerous in combination with anticoagulants and theophylline (asthma drugs), and dangerous with anticonvulsants, antifungals and heart drugs such as calcium antagonists and quinidines. Both Tagamet and its rival drug Zantac have both been shown to reduce the effectiveness of antifungal drugs such as Nizoral.
Antidepressants: Monoamine oxidase inhibitors (MAOIs) cause severe reactions when consumed with foods containing a substance called tyramine. This interaction can result in very high blood pressure from everyday foods such as mature cheese, avocado, liver and Chianti. It has caused deaths.
Another dangerous combination is MAOIs taken with the new selective serotonin reuptake inhibitor antidepressants (SSRIs), such as Prozac. SSRIs have a different mode of action from MAOIs in that they block the uptake of serotonin. In combination, they have produced the frequently lethal "serotonin syndrome".
Even the newer and apparently safer MAOIs, such as moclobemide, have caused serious interactions with both SSRIs and tricyclic antidepressants (J Intern Med, 1993; 233: 113-5). Some cough remedies are also dangerous with MAOIs.
Antihistamines: Astemizole and terfenadine are potentially dangerous with some antifungal drugs, macrolide antibiotics and the dyspepsia drug cisapride. There are few warning signs of danger dizziness, fainting and shortness of breath are often the first inkling of an interaction problem, which can result in irregular heart rhythms and even cardiac arrest. Hismanal and Seldane also interact with each other, potentially lethally.
Arthritis drugs (NSAIDs): These can be lethal in combination with another arthritis drug, methotrexate and the anticoagulant warfarin. There can be dangerous interactions with heart drugs such as beta blockers, ACE inhibitors and diuretics. Methotrexate can also be lethal with antibiotics.
Antibiotics: Co-trimoxazole can be lethal with warfarin and dangerous with many diabetes drugs. Macrolides can be lethal with the cholesterol drugs Mevacor, Pravachol and Zocor, the antihistamines Hismanal and Seldane and the anticonvulsant Tegretol.
Ritalin: This drug (methylphenidate) is increasingly being prescribed for hyperactive children, although in the US it appears to be given to any "problem child", so that now an estimated one in five children are on it. In the US, the antidepressant imipramine (Tofranil) has been co-prescribed with Ritalin, but anecdotal reports suggest that the combination can lead to dysfunctional behaviour. There have also been concerns about interactions with the anticonvulsant phenytoin (Dilantin), leading to exacerbated side effects.