If a doctor prescribes a medicine of a particular strength, but that exact strength is not available and a lower-strength tablet with the exact same salt/composition is available, can I take two tablets of the lower strength instead of one tablet of the prescribed higher strength? For example, if the prescribed strength is 40 mg and only 20 mg tablets with the exact same salt are available, would taking two 20 mg tablets be equivalent to taking one 40 mg tablet? Is this generally safe and equivalent, or are there any differences in absorption, formulation, or dosing that could make this inadvisable?