Hello, several drugs are altered by exposure to light in relation with their photodegradation or other chemical reactions that affect drug stability. I would be prudent and would not use them since some products may be toxic.
The following reference
King AR. Light-sensitive oral prescription drugs. Hosp Pharm. 2009;44(12):1112–1114
Yes I also agree,however all do not result in toxic substance,nontheless it is better not to use light exposed drugs when such instructions are given ,light reacts as fast as in femto seconds
Since it is impossible to know the degree to which the quality of the product has been compromised I would not use it at all. Qualitative testing would have to be conducted otherwise.
This is very important issue especially in context to developing nations where resources and facilities are scarce! It is difficult to quantify the extent of damage caused due to exposure to light. In some cases, it still can be used but in other, may not. So, it is advisable not to use it.
In developing countries one of the major issues is linked to falsified drugs and fake medicine which results in important health issues in those countries.