Every clinic has a washout period during which you are not allowed to do a study at the same clinic or any other clinic. 

The minimum period is 30 days since last dose, however some washouts go based on last study procedure (such as out patient visits).

The washout period may vary from study to study depending on the half-life of the drug.

Certain studies like radio-labeled drugs will have a 1 year washout before you can do another radio-labeled drug. 

In addition to the clinic's washout policy, the sponsor may impose a longer washout period.

Some studies will have a longer washout before you can sign up. 

After completing a study with many blood collections, you may want to take some iron supplements to help rebuild your blood count.

I recommend natural vitamins found in foods like mussels, fish, beans, leafy greens, flax seed and other foods high in iron content. 

Supplements can take longer to absorb than natural vitamins found in foods. 

You should also intake a constant amount of water.  I recommend the daily allowance of 2 to 2 and a half liters a day.  This should be spread throughout the day.  It is important to keep hydrated at all times.  If you feel thirsty, then you are already dehydrated.