Some species in the Ephedra Genus, such as Ephedra Viridis for example, contain no alkaloids so they can be safely used in the form of dietary supplements by people taking other medications, as no specific interactions between the herb and conventional drugs have been reported.

So in this first case the answer is yes, you can use Ephedra if taking medications, as long as you opt for products listed in the dietary supplements category and containing no ephedrine or pseudoephedrine.

On the other hand, if you intend to use Ephedrine when taking medications, you have to be very careful as there are several drugs that interact with this alkaloid:

Asthma medications and drugs used for bronchial congestion, such as Theophylline (T-Phyl, Uniphyl, Aerolate);
Aspirin and blood-thinning drugs, that can lead to increased bleeding when taken in combination with Ephedrine by sensitive patients;
Amphetamines and derivatives such as dextroamphetamine (Adderall), used for narcolepsy and attention deficit hyperactivity disorder, that increase blood pressure and heart rate;
Antidepressants, such as MAOIs (monoamine oxidase inhibitors: phenelzine, trancylcypromine) and tricyclic drugs (clomipramine, amitriptyline, nortriptyline, imipramine), that increase the risk of stroke;
Blood pressure medicines, such as clonidine, that are supposed to treat hypertension;
Diabetes medicines, such as insulin and metformin;
Narcotics like codeine, that are prescribed for pain and in some cases for cough;
Caffeine, herbs and drugs containing this stimulant (guarana, kola nut, green tea), that increase blood pressure and have additive effects;

These potential interactions of Ephedrine alkaloids are well known and most of the times mentioned on the labels of drugs and supplements containing this ingredient, so you should not use Ephedrine if taking medications in the list above.

And again (important): You should always consult with your doctor before Ephedra/Ephedrine usage if you are taking medications!